dt-bindings: sram: Convert SRAM bindings to json-schema

Convert generic mmio-sram bindings to DT schema format using
json-schema.  Require the address/size cells to be 1, not equal to root
node.  This also fixes the check for clocks property to be in main root
node instead of children.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>