arm64: dts: rockchip: add rs485 support on uart5 of px30-ringneck-haikou

A hardware switch can set the rs485 transceiver into half or full duplex
mode.

Switching to the half-duplex mode requires the user to enable em485 on
uart5 using ioctl, DE/RE are both connected to GPIO0_B5 which is the
RTS signal for uart0. Implement GPIO0_B5 as rts-gpios with RTS_ON_SEND
option enabled (default) so that driver mode gets enabled while sending
(RTS high) and receiver mode gets enabled while not sending (RTS low).

In full-duplex mode (em485 is disabled), DE is connected to GPIO0_B5 and
RE is grounded (enabled). Since GPIO0_B5 is implemented as rts-gpios, the
driver mode gets enabled whenever we want to send something and RE is not
affected (always enabled) in this case by the state of RTS.

Signed-off-by: Farouk Bouabid <farouk.bouabid@theobroma-systems.com>
Link: https://lore.kernel.org/r/20240208-dev-rx-enable-v6-2-39e68e17a339@theobroma-systems.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/px30-ringneck-haikou.dts

@@ -227,6 +227,7 @@
 
 &uart5 {
 	pinctrl-0 = <&uart5_xfer>;
+	rts-gpios = <&gpio0 RK_PB5 GPIO_ACTIVE_HIGH>;
 	status = "okay";
 };