dt-bindings: sram: Add 'clocks' as an optional property

Some SoCs (like i.MX53) need to specify the SRAM clock in the
device tree via the clocks property.

Add an entry to the optional property section.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>