ARM: rockchip: rk3188: Remove the pinctrl setup and enable uart at SPL

When the boot ROM sets up MMC we don't need to do it again. Remove the
MMC setup code entirely, but we also need to enable uart for debug message.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>