UART1 can optionally be used with additional CTS/RTS pins.
The pinctrl driver has an extra "u1ctsrts_a_1" pin group for them.

Add a new pin configuration to configure them correctly if needed.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20191125122256.53482-4-stephan@gerhold.netSigned-off-by: Linus Walleij <linus.walleij@linaro.org>