pinctrl: cygnus: Optional DT property to support pin mappings

If GPIO controller's pins are muxed, pin-controller subsystem
need to be intimated by defining mapping between gpio and
pinmux controller. This patch adds required properties to
define this mapping via DT.

Signed-off-by: Pramod Kumar <pramodku@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>