dt-bindings: mfd: aspeed-lpc: Make parameter optional

Memory-region and flash phandle is not a required parameter, it is
optional to describe in device tree and needed only use basis.

Signed-off-by: Vijay Khemka <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/aspeed-lpc.txt

···
 - clocks:	contains a phandle to the syscon node describing the clocks.
 		There should then be one cell representing the clock to use
 
+Optional properties:
+
 - memory-region: A phandle to a reserved_memory region to be used for the LPC
 		to AHB mapping