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 <venture@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>

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

@@ -18,6 +18,7 @@ Required properties:
 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