dt-bindings: misc: Document reg for aspeed, p2a-ctrl nodes

The P2A controller node sits under a syscon device, and can assume
offsets from the base of the syscon based on the compatible. However,
for devicetree correctness allow a reg property to be specified, which
an associated driver may choose to use to discover associated resources.

Cc: Patrick Venture <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Andrew Jeffery <[e-mail]>
Reviewed-by: Joel Stanley <[e-mail]>
Signed-off-by: Joel Stanley <[e-mail]>

Documentation/devicetree/bindings/misc/aspeed-p2a-ctrl.txt

···
 Optional properties:
 ===================
 
+- reg: A hint for the memory regions associated with the P2A controller
 - memory-region: A phandle to a reserved_memory region to be used for the PCI
 		to AHB mapping