arm64: dts: imx8mp-phyboard-pollux-rdk: correct uart pad settings

BIT3 and BIT0 are reserved bits, should not touch.

Fixes: 846f7528

 ("arm64: dts: imx8mp-phyboard-pollux-rdk: Change debug UART")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>