arm64: dts: rockchip: Add pinctrl for UART0 to rk3308-rock-pi-s

UAR0 CTS/RTS is not wired to any pin and is not used for the default
serial console use of UART0 on ROCK Pi S.

Override the SoC defined pinctrl props to limit configuration of the
two xfer pins wired to one of the GPIO pin headers.

Fixes: 2e04c25b ("arm64: dts: rockchip: add ROCK Pi S DTS support")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Link: https://lore.kernel.org/r/20240521211029.1236094-6-jonas@kwiboo.seSigned-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3308-rock-pi-s.dts

@@ -286,6 +286,8 @@ u2phy_otg: otg-port {
 };
 
 &uart0 {
+	pinctrl-names = "default";
+	pinctrl-0 = <&uart0_xfer>;
 	status = "okay";
 };