ARM: tegra: define DT bindings for and instantiate RTC

The Tegra RTC maintains seconds and milliseconds counters, and five alarm
registers. The alarms and other interrupts may wake the system from
low-power state.

Define a DT binding for this HW module, and add the module into the Tegra
device tree files.

Signed-off-by: Stephen Warren <[e-mail]>

Documentation/devicetree/bindings/rtc/nvidia,tegra20-rtc.txt

···
+NVIDIA Tegra20 real-time clock
+
+The Tegra RTC maintains seconds and milliseconds counters, and five alarm
+registers. The alarms and other interrupts may wake the system from low-power
+state.
+
+Required properties:
+
+- compatible : should be "nvidia,tegra20-rtc".
+- reg : Specifies base physical address and size of the registers.
+- interrupts : A single interrupt specifier.
+
+Example:
+
+timer {
+	compatible = "nvidia,tegra20-rtc";
+	reg = <0x7000e000 0x100>;
+	interrupts = <0 2 0x04>;
+};

arch/arm/boot/dts/tegra20.dtsi

···
 		#pwm-cells = <2>;
 	};
 
+	rtc {
+		compatible = "nvidia,tegra20-rtc";
+		reg = <0x7000e000 0x100>;
+		interrupts = <0 2 0x04>;
+	};
+
 	i2c@7000c000 {
 		compatible = "nvidia,tegra20-i2c";
 		reg = <0x7000c000 0x100>;

arch/arm/boot/dts/tegra30.dtsi

···
 		#pwm-cells = <2>;
 	};
 
+	rtc {
+		compatible = "nvidia,tegra30-rtc", "nvidia,tegra20-rtc";
+		reg = <0x7000e000 0x100>;
+		interrupts = <0 2 0x04>;
+	};
+
 	i2c@7000c000 {
 		compatible =  "nvidia,tegra30-i2c", "nvidia,tegra20-i2c";
 		reg = <0x7000c000 0x100>;