dt-bindings: memory: Add bindings for imx8m ddr controller

Add devicetree bindings for the i.MX DDR Controller on imx8m series
chips. It supports dynamic frequency switching between multiple data
rates and this is exposed to Linux via the devfreq subsystem.
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

Documentation/devicetree/bindings/memory-controllers/fsl/imx8m-ddrc.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/memory-controllers/fsl/imx8m-ddrc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: i.MX8M DDR Controller
+
+maintainers:
+  - Leonard Crestez <leonard.crestez@nxp.com>
+
+description:
+  The DDRC block is integrated in i.MX8M for interfacing with DDR based
+  memories.
+
+  It supports switching between different frequencies at runtime but during
+  this process RAM itself becomes briefly inaccessible so actual frequency
+  switching is implemented by TF-A code which runs from a SRAM area.
+
+  The Linux driver for the DDRC doesn't even map registers (they're included
+  for the sake of "describing hardware"), it mostly just exposes firmware
+  capabilities through standard Linux mechanism like devfreq and OPP tables.
+
+properties:
+  compatible:
+    items:
+      - enum:
+        - fsl,imx8mn-ddrc
+        - fsl,imx8mm-ddrc
+        - fsl,imx8mq-ddrc
+      - const: fsl,imx8m-ddrc
+
+  reg:
+    maxItems: 1
+    description:
+      Base address and size of DDRC CTL area.
+      This is not currently mapped by the imx8m-ddrc driver.
+
+  clocks:
+    maxItems: 4
+
+  clock-names:
+    items:
+      - const: core
+      - const: pll
+      - const: alt
+      - const: apb
+
+  operating-points-v2: true
+  opp-table: true
+
+required:
+  - reg
+  - compatible
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/imx8mm-clock.h>
+    ddrc: memory-controller@3d400000 {
+        compatible = "fsl,imx8mm-ddrc", "fsl,imx8m-ddrc";
+        reg = <0x3d400000 0x400000>;
+        clock-names = "core", "pll", "alt", "apb";
+        clocks = <&clk IMX8MM_CLK_DRAM_CORE>,
+                 <&clk IMX8MM_DRAM_PLL>,
+                 <&clk IMX8MM_CLK_DRAM_ALT>,
+                 <&clk IMX8MM_CLK_DRAM_APB>;
+        operating-points-v2 = <&ddrc_opp_table>;
+    };