arm64: dts: imx8mm-venice-gw72xx-0x: add dt overlays for serial modes

The imx8mm-venice-gw72xx-0x som+baseboard combination has a multi-protocol
RS-232/RS-485/RS-422 transceiver to an off-board connector which
can be configured in a number of ways via UART and GPIO configuration.

The default configuration per the imx8mm-venice-gw72xx-0x dts is for
UART2 TX/RX and UART4 TX/RX to be available as RS-232:
 J15.1 UART2 TX out
 J15.2 UART2 RX in
 J15.3 UART4 TX out
 J15.4 UART4 RX in
 J15.5 GND

Add dt overlays to allow additional the modes of operation:

rs232-rts (UART2 RS-232 with RTS/CTS hardware flow control)
 J15.1 TX out
 J15.2 RX in
 J15.3 RTS out
 J15.4 CTS in
 J15.5 GND

rs485 (UART2 RS-485 half duplex)
 J15.1 TXRX-
 J15.2 N/C
 J15.3 TXRX+
 J15.4 N/C
 J15.5 GND

rs422 (UART2 RS-422 full duplex)
 J15.1 TX-
 J15.2 RX+
 J15.3 TX+
 J15.4 RX-
 J15.5 GND

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>