ARM: dt: tegra: Add labels for I2S controllers

This will allow the sound node to refer to the I2S controllers by name
when creating phandles.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/boot/dts/tegra20.dtsi

@@ -65,14 +65,14 @@ i2c@7000d000 {
 		interrupts = < 0 53 0x04 >;
 	};
 
-	i2s@70002800 {
+	tegra_i2s1: i2s@70002800 {
 		compatible = "nvidia,tegra20-i2s";
 		reg = <0x70002800 0x200>;
 		interrupts = < 0 13 0x04 >;
 		nvidia,dma-request-selector = < &apbdma 2 >;
 	};
 
-	i2s@70002a00 {
+	tegra_i2s2: i2s@70002a00 {
 		compatible = "nvidia,tegra20-i2s";
 		reg = <0x70002a00 0x200>;
 		interrupts = < 0 3 0x04 >;