ARM: tegra: dts: Add pwm label

PWM devices can be referenced in the DT by phandle and per-chip index.
In order for this to work properly, the PWM controller needs to have a
label attached to it.
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: NStephen Warren <swarren@nvidia.com>

arch/arm/boot/dts/tegra20.dtsi

@@ -123,7 +123,7 @@
 		status = "disabled";
 	};
 
-	pwm {
+	pwm: pwm {
 		compatible = "nvidia,tegra20-pwm";
 		reg = <0x7000a000 0x100>;
 		#pwm-cells = <2>;

arch/arm/boot/dts/tegra30.dtsi

@@ -117,7 +117,7 @@
 		status = "disabled";
 	};
 
-	pwm {
+	pwm: pwm {
 		compatible = "nvidia,tegra30-pwm", "nvidia,tegra20-pwm";
 		reg = <0x7000a000 0x100>;
 		#pwm-cells = <2>;