dt-bindings: memory: Add Tegra186 memory subsystem

The NVIDIA Tegra186 SoC contains a memory subsystem composed of the
memory controller and the external memory controller. The memory
controller provides interfaces for the memory clients to access the
memory. Accesses can be either bounced through the SMMU for IOVA
translation or directly to the EMC.

The bulk of the programming of the external memory controller happens
through interfaces exposed by the BPMP. Describe this relationship by
adding a phandle reference to the BPMP to the EMC node.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/memory-controllers/nvidia,tegra186-mc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/memory-controllers/nvidia,tegra186-mc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NVIDIA Tegra186 (and later) SoC Memory Controller
+
+maintainers:
+  - Jon Hunter <jonathanh@nvidia.com>
+  - Thierry Reding <thierry.reding@gmail.com>
+
+description: |
+  The NVIDIA Tegra186 SoC features a 128 bit memory controller that is split
+  into four 32 bit channels to support LPDDR4 with x16 subpartitions. The MC
+  handles memory requests for 40-bit virtual addresses from internal clients
+  and arbitrates among them to allocate memory bandwidth.
+
+  Up to 15 GiB of physical memory can be supported. Security features such as
+  encryption of traffic to and from DRAM via general security apertures are
+  available for video and other secure applications, as well as DRAM ECC for
+  automotive safety applications (single bit error correction and double bit
+  error detection).
+
+properties:
+  $nodename:
+    pattern: "^memory-controller@[0-9a-f]+$"
+
+  compatible:
+    items:
+      - enum:
+          - nvidia,tegra186-mc
+          - nvidia,tegra194-mc
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  "#address-cells":
+    const: 2
+
+  "#size-cells":
+    const: 2
+
+  ranges: true
+
+  dma-ranges: true
+
+patternProperties:
+  "^external-memory-controller@[0-9a-f]+$":
+    description:
+      The bulk of the work involved in controlling the external memory
+      controller on NVIDIA Tegra186 and later is performed on the BPMP. This
+      coprocessor exposes the EMC clock that is used to set the frequency at
+      which the external memory is clocked and a remote procedure call that
+      can be used to obtain the set of available frequencies.
+    type: object
+    properties:
+      compatible:
+        items:
+          - enum:
+              - nvidia,tegra186-emc
+              - nvidia,tegra194-emc
+
+      reg:
+        maxItems: 1
+
+      interrupts:
+        maxItems: 1
+
+      clocks:
+        items:
+          - description: external memory clock
+
+      clock-names:
+        items:
+          - const: emc
+
+      nvidia,bpmp:
+        $ref: /schemas/types.yaml#/definitions/phandle
+        description:
+          phandle of the node representing the BPMP
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - "#address-cells"
+  - "#size-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/tegra186-clock.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+
+    memory-controller@2c00000 {
+        compatible = "nvidia,tegra186-mc";
+        reg = <0x0 0x02c00000 0x0 0xb0000>;
+        interrupts = <GIC_SPI 223 IRQ_TYPE_LEVEL_HIGH>;
+
+        #address-cells = <2>;
+        #size-cells = <2>;
+
+        ranges = <0x0 0x02c00000 0x02c00000 0x0 0xb0000>;
+
+        /*
+         * Memory clients have access to all 40 bits that the memory
+         * controller can address.
+         */
+        dma-ranges = <0x0 0x0 0x0 0x0 0x100 0x0>;
+
+        external-memory-controller@2c60000 {
+            compatible = "nvidia,tegra186-emc";
+            reg = <0x0 0x02c60000 0x0 0x50000>;
+            interrupts = <GIC_SPI 224 IRQ_TYPE_LEVEL_HIGH>;
+            clocks = <&bpmp TEGRA186_CLK_EMC>;
+            clock-names = "emc";
+
+            nvidia,bpmp = <&bpmp>;
+        };
+    };
+
+    bpmp: bpmp {
+        compatible = "nvidia,tegra186-bpmp";
+        #clock-cells = <1>;
+    };