arm64: dts: imx8mm-kontron: Use GPIO for RS485 transceiver control

arm64: dts: imx8mm-kontron: Use GPIO for RS485 transceiver control

For this IP the correct control of the CTS signal for transceiver direction
switching is difficult and - maybe also buggy - in the driver. Especially
the bootup requires special handling for most hardware implementations.

Therefore we simply use a GPIO now, which is fully under software control
and which is not problematic on bootup.

Signed-off-by: Eberhard Stoll <eberhard.stoll@kontron.de>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>