arm64: dts: imx8mm-phyboard-polis: Set debug uart muxing to 0x140

Set Pull Resistors Enable bit to put signal into a defined state.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Cem Tenruh <c.tenruh@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>