dt-bindings: pinctrl: aspeed: Add reg property as a hint

The pinctrl 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: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>