watchdog: bindings: Supply knowledge of a third supported device - clocksource

Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/watchdog/st_lpc_wdt.txt

 STMicroelectronics Low Power Controller (LPC) - Watchdog
 ========================================================
 
-LPC currently supports Watchdog OR Real Time Clock functionality.
+LPC currently supports Watchdog OR Real Time Clock OR Clocksource
+functionality.
 
 [See: ../rtc/rtc-st-lpc.txt for RTC options]
+[See: ../timer/st,stih407-lpc for Clocksource options]
 
 Required properties
 
@@ -12,9 +14,11 @@ Required properties
 - 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.
+- 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.
 
 Required properties [watchdog mode]