clocksource: bindings: Provide bindings for ST's LPC Clocksource 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 Clocksource mode.
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/timer/st,stih407-lpc

+STMicroelectronics Low Power Controller (LPC) - Clocksource
+===========================================================
+
+LPC currently supports Watchdog OR Real Time Clock OR Clocksource
+functionality.
+
+[See: ../watchdog/st_lpc_wdt.txt for Watchdog options]
+[See: ../rtc/rtc-st-lpc.txt for RTC options]
+
+Required properties
+
+- compatible   : Must be: "st,stih407-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 three modes:
+                  ST_LPC_MODE_RTC    [0]
+                  ST_LPC_MODE_WDT    [1]
+                  ST_LPC_MODE_CLKSRC [2]
+		 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_CLKSRC>;
+       };