dt-bindings: sc16is7xx: Add property to change GPIO function

dt-bindings: sc16is7xx: Add property to change GPIO function

Some variants in this series of UART controllers have GPIO pins that
are shared between GPIO and modem control lines.

The pin mux mode (GPIO or modem control lines) can be set for each
ports (channels) supported by the variant.

This adds a property to the device tree to set the GPIO pin mux to
modem control lines on selected ports if needed.

Cc: stable@vger.kernel.org # 6.1.x
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Lech Perczak <lech.perczak@camlingroup.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230807214556.540627-4-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>