ARM: dts: sun7i: add pin muxing options for UART2

UART2 is used on CubieTruck to connect to the Bluetooth module.
Add the pin set used in this case.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/boot/dts/sun7i-a20.dtsi

@@ -381,6 +381,13 @@ uart0_pins_a: uart0@0 {
 				allwinner,pull = <0>;
 			};
 
+			uart2_pins_a: uart2@0 {
+				allwinner,pins = "PI16", "PI17", "PI18", "PI19";
+				allwinner,function = "uart2";
+				allwinner,drive = <0>;
+				allwinner,pull = <0>;
+			};
+
 			uart6_pins_a: uart6@0 {
 				allwinner,pins = "PI12", "PI13";
 				allwinner,function = "uart6";