rockchip: rk3288: Add pinctrl support for the gmac ethernet interface

Add support for the gmac ethernet interface to pinctrl. This hardcodes
the setup to match that of the firefly and Radxa Rock2 boards, using the
RGMII phy mode for gmac interface and GPIO4B0 as the phy reset GPIO.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Romain Perier <romain.perier@collabora.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>