mfd: dt-bindings: Provide human readable define for Clocksource mode

ST's Low Power Controller can now operate in three supported modes;
Watchdog, Real Time Clock and most recently as a Clocksource.  This new
define will allow the LPC IP to be configured for Clocksource from DT.
Signed-off-by: Lee Jones <lee.jones@linaro.org>

include/dt-bindings/mfd/st-lpc.h

@@ -11,5 +11,6 @@
 
 #define ST_LPC_MODE_RTC		0
 #define ST_LPC_MODE_WDT		1
+#define ST_LPC_MODE_CLKSRC	2
 
 #endif /* __DT_BINDINGS_ST_LPC_H__ */