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UART2 and UART4 can used in following combination,

2 x RS232:
- UART2 and UART4 without flow control
- MUX selection GPIO3_20 must be held low

1 x RS232 + 1 x RS485:
- UART2 - RS232
- UART4 - RS485
- MUX selection GPIO3_20 must be held high
- RE/DE for RS485 controlled with GPIO3_25

1 x RS232 with flow control:
- UART2 - RS232
- MUX selection GPIO3_20 must be held low
Signed-off-by: Parthiban Nallathambi <parthiban@linumiz.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>