rockchip: remove the hard coded uart iomux setting for px5 evb

As the debug uart is marked as dm-pre-reloc, the pinctrl driver
will handle the correct iomux setting.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>