ARM: mvebu: Add spdif-pins mpp pins for Armada 38x

S/PDIF function on Armada 38x uses only mpp51 pin. So add spdif-pins mpp
pins section for it. It is needed for boards without i2s.
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

arch/arm/boot/dts/armada-38x.dtsi

@@ -296,6 +296,11 @@ i2s_pins: i2s-pins {
 						       "mpp52", "mpp53";
 					marvell,function = "audio";
 				};
+
+				spdif_pins: spdif-pins {
+					marvell,pins = "mpp51";
+					marvell,function = "audio";
+				};
 			};
 
 			gpio0: gpio@18100 {