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: 2e04c25b1320 ("arm64: dts: rockchip: add ROCK Pi S DTS support")
Signed-off-by: Jonas Karlman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Heiko Stuebner <[email protected]>

[ upstream commit: 7affb86ef62581e3475ce3e0a7640da1f2ee29f8 ]

(cherry picked from commit 9c72cd5fa9f971be8ebbc1f43bd74a72e33db2fa)
Reviewed-by: Kever Yang <[email protected]>

dts/upstream/src/arm64/rockchip/rk3308-rock-pi-s.dts

@@ -294,6 +294,8 @@
 };
 
 &uart0 {
+	pinctrl-names = "default";
+	pinctrl-0 = <&uart0_xfer>;
 	status = "okay";
 };