dt-bindings: rtc: document the rtc-sh bindings

Add the binding documentation for rtc-sh which is an RTC for
SuperH and RZ/A SoCs.
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/rtc-sh.txt

+* Real Time Clock for Renesas SH and ARM SoCs
+
+Required properties:
+- compatible: Should be "renesas,r7s72100-rtc" and "renesas,sh-rtc" as a
+  fallback.
+- reg: physical base address and length of memory mapped region.
+- interrupts: 3 interrupts for alarm, period, and carry.
+- interrupt-names: The interrupts should be labeled as "alarm", "period", and
+  "carry".
+- clocks: The functional clock source for the RTC controller must be listed
+  first (if exists). Additionally, potential clock counting sources are to be
+  listed.
+- clock-names: The functional clock must be labeled as "fck". Other clocks
+  may be named in accordance to the SoC hardware manuals.
+
+
+Example:
+rtc: rtc@fcff1000 {
+	compatible = "renesas,r7s72100-rtc", "renesas,sh-rtc";
+	reg = <0xfcff1000 0x2e>;
+	interrupts = <GIC_SPI 276 IRQ_TYPE_EDGE_RISING
+		      GIC_SPI 277 IRQ_TYPE_EDGE_RISING
+		      GIC_SPI 278 IRQ_TYPE_EDGE_RISING>;
+	interrupt-names = "alarm", "period", "carry";
+	clocks = <&mstp6_clks R7S72100_CLK_RTC>, <&rtc_x1_clk>,
+		 <&rtc_x3_clk>, <&extal_clk>;
+	clock-names = "fck", "rtc_x1", "rtc_x3", "extal";
+};