rockchip: pinctrl: rk3399: add GMAC (RGMII only) support

rockchip: pinctrl: rk3399: add GMAC (RGMII only) support

To add GMAC (Gigabit Ethernet) support (limited to RGMII only at this
point), we need support for additional pin-configuration.  This commit
adds the pinctrl support for GMAC in RGMII signalling mode:
 * adds a PERIPH_ID_GMAC and the mapping from IRQ number to PERIPH_ID
 * adds the required defines (in the GRF support) for configuring the
   GPIOC pins for RGMII
 * configures the RGMII pins (in GPIOC) when requested via pinctrl

X-AffectedPlatforms: RK3399-Q7
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>