dt/bindings: Add bindings for the PIC32 real time clock

Document the devicetree bindings for the real time clock found
on Microchip PIC32 class devices.
Signed-off-by: NJoshua Henderson <joshua.henderson@microchip.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/microchip,pic32-rtc.txt

+* Microchip PIC32 Real Time Clock and Calendar
+
+The RTCC keeps time in hours, minutes, and seconds, and one half second. It
+provides a calendar in weekday, date, month, and year. It also provides a
+configurable alarm.
+
+Required properties:
+- compatible: should be: "microchip,pic32mzda-rtc"
+- reg: physical base address of the controller and length of memory mapped
+    region.
+- interrupts: RTC alarm/event interrupt
+- clocks: clock phandle
+
+Example:
+
+	rtc: rtc@1f8c0000 {
+		compatible = "microchip,pic32mzda-rtc";
+		reg = <0x1f8c0000 0x60>;
+		interrupts = <166 IRQ_TYPE_EDGE_RISING>;
+		clocks = <&PBCLK6>;
+	};