dt-bindings: memory-controllers: add canaan k210 sram controller

The k210 U-Boot port has been using the clocks defined in the
devicetree to bring up the board's SRAM, but this violates the
dt-schema. As such, move the clocks to a dedicated node with
the same compatible string & document it.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220705215213.1802496-5-mail@conchuod.ieSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/memory-controllers/canaan,k210-sram.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/memory-controllers/canaan,k210-sram.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Canaan K210 SRAM memory controller
+
+description:
+  The Canaan K210 SRAM memory controller is responsible for the system's 8 MiB
+  of SRAM. The controller is initialised by the bootloader, which configures
+  its clocks, before OS bringup.
+
+maintainers:
+  - Conor Dooley <conor@kernel.org>
+
+properties:
+  compatible:
+    enum:
+      - canaan,k210-sram
+
+  clocks:
+    minItems: 1
+    items:
+      - description: sram0 clock
+      - description: sram1 clock
+      - description: aisram clock
+
+  clock-names:
+    minItems: 1
+    items:
+      - const: sram0
+      - const: sram1
+      - const: aisram
+
+required:
+  - compatible
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/k210-clk.h>
+    memory-controller {
+        compatible = "canaan,k210-sram";
+        clocks = <&sysclk K210_CLK_SRAM0>,
+                 <&sysclk K210_CLK_SRAM1>,
+                 <&sysclk K210_CLK_AI>;
+        clock-names = "sram0", "sram1", "aisram";
+    };