dt/bindings: Add bindings for Micro Crystal rv8803

... and Epson RX8900 real time clock
Signed-off-by: NOleksij Rempel <fixed-term.Oleksij.Rempel@de.bosch.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/epson,rx8900.txt

+Real Time Clock driver for:
+  - Epson RX8900
+  - Micro Crystal rv8803
+
+Required properties:
+- compatible: should be: "microcrystal,rv8803" or "epson,rx8900"
+- reg : the I2C address of the device for I2C
+
+Optional properties:
+- epson,vdet-disable : boolean, if present will disable voltage detector.
+  Should be set if no backup battery is used.
+- trickle-diode-disable : boolean, if present will disable internal trickle
+  charger diode
+
+Example:
+
+	rtc: rtc@32 {
+		compatible = "epson,rx8900"
+		reg = <0x32>;
+		epson,vdet-disable;
+		trickle-diode-disable;
+	};