ARM: mstar: Add binding details for mstar,l3bridge

This adds a YAML description of the l3bridge node needed by the
platform code for the MStar/SigmaStar Armv7 SoCs.
Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/misc/mstar,l3bridge.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2020 thingy.jp.
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/misc/mstar,l3bridge.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: MStar/SigmaStar Armv7 SoC l3bridge
+
+maintainers:
+  - Daniel Palmer <daniel@thingy.jp>
+
+description: |
+  MStar/SigmaStar's Armv7 SoCs have a pipeline in the interface
+  between the CPU and memory. This means that before DMA capable
+  devices are allowed to run the pipeline must be flushed to ensure
+  everything is in memory.
+
+  The l3bridge region contains registers that allow such a flush
+  to be triggered.
+
+  This node is used by the platform code to find where the registers
+  are and install a barrier that triggers the required pipeline flush.
+
+properties:
+  compatible:
+    items:
+      - const: mstar,l3bridge
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    l3bridge: l3bridge@1f204400 {
+        compatible = "mstar,l3bridge";
+        reg = <0x1f204400 0x200>;
+    };