arm: dts: mt7622: remove default pinctrl of uart0

Currently u-boot running on mt7622 will print an warning log at beginning:
> serial_mtk serial@11002000: pinctrl_select_state_full: uclass_get_device_by_phandle_id: err=-19

It turns out that the pinctrl uclass can't work properly in board_f stage.

Since the uart0 is the default UART device used by bootrom, and will be
initialized in both bootrom and tf-a bl2. It's ok not to setup pinctrl for
uart0 in u-boot.

This patch removes the default pinctrl of uart0 to suppress the unwanted
warning.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>