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 <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/sram/sram.txt

···
 		 manipulation of the page attributes.
 - label : the name for the reserved partition, if omitted, the label
           is taken from the node name excluding the unit address.
+- clocks : a list of phandle and clock specifier pair that controls the
+	   single SRAM clock.
 
 Example: