ARM: sun7i: DT: Add UART muxing options to the DTSI

The UARTs on the A20 can be muxed to several pins. Add a few options to
the DTSI so that we can start using them in the boards' DT.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

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

@@ -71,6 +71,27 @@ pio: pinctrl@01c20800 {
 			#address-cells = <1>;
 			#size-cells = <0>;
 			#gpio-cells = <3>;
+
+			uart0_pins_a: uart0@0 {
+				allwinner,pins = "PB22", "PB23";
+				allwinner,function = "uart0";
+				allwinner,drive = <0>;
+				allwinner,pull = <0>;
+			};
+
+			uart6_pins_a: uart6@0 {
+				allwinner,pins = "PI12", "PI13";
+				allwinner,function = "uart6";
+				allwinner,drive = <0>;
+				allwinner,pull = <0>;
+			};
+
+			uart7_pins_a: uart7@0 {
+				allwinner,pins = "PI20", "PI21";
+				allwinner,function = "uart7";
+				allwinner,drive = <0>;
+				allwinner,pull = <0>;
+			};
 		};
 
 		timer@01c20c00 {