rtc: bindings: Provide ST bindings for ST's LPC RTC device

On current ST platforms the LPC controls a number of functions including
Watchdog and Real Time Clock.  This patch provides the bindings used to
configure LPC in RTC mode.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/rtc/rtc-st-lpc.txt

+STMicroelectronics Low Power Controller (LPC) - RTC
+===================================================
+
+LPC currently supports Watchdog OR Real Time Clock functionality.
+
+[See: ../watchdog/st_lpc_wdt.txt for Watchdog options]
+
+Required properties
+
+- compatible 	: Must be one of: "st,stih407-lpc" "st,stih416-lpc"
+				  "st,stih415-lpc" "st,stid127-lpc"
+- reg		: LPC registers base address + size
+- interrupts    : LPC interrupt line number and associated flags
+- clocks	: Clock used by LPC device (See: ../clock/clock-bindings.txt)
+- st,lpc-mode	: The LPC can run either one of two modes ST_LPC_MODE_RTC [0] or
+		  ST_LPC_MODE_WDT [1].  One (and only one) mode must be
+		  selected.
+
+Example:
+	lpc@fde05000 {
+		compatible	= "st,stih407-lpc";
+		reg		= <0xfde05000 0x1000>;
+		clocks 		= <&clk_s_d3_flexgen CLK_LPC_0>;
+		st,lpc-mode	= <ST_LPC_MODE_RTC>;
+	};