rockchip: pinctrl: rk3368: add pinctrl for SDMMC0

rockchip: pinctrl: rk3368: add pinctrl for SDMMC0

Apparently, our earlier assumption that the BROM will always set up
the iomux for SDcard communication does not always hold true: when
booting U-Boot from the on-module (on the RK3368-uQ7) eMMC, the SDcard
pins are not set up and need to be configured by the pinctrl driver to
allow SD card access.

This change implements support for setting up the SDMMC pins in
pinctrl for the RK3368.

Reported-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>