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

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.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>