arm64: dts: imx8mp-phyboard-pollux: Add support for RS232/RS485

Add UART2 for RS232/RS485 support.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
(Updated the node by not setting the reserved bits(BIT 0 and BIT 3)
and enabled internal pullup for RX and TX.)
Signed-off-by: Yashwanth Varakala <y.varakala@phytec.de>
Signed-off-by: Cem Tenruh <c.tenruh@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>