clocksource: bindings: Provide bindings for ST's LPC Clocksource device

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>;

       };