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>