rockchip: pinctrl: rk3368: add support for configuring the MMC pins

rockchip: pinctrl: rk3368: add support for configuring the MMC pins

The RK3368 has two SD/MMC controllers that can be used from U-Boot
both during SPL and for booting an OS from the full bootloader stage.
While both are configured to (mostly) sensible settings from the BROM,
additional configuration for the MMC controller is needed to configure
it to 8bit mode.

This adds pinctrl support for the MMC controller.

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