clockevents/drivers/Kconfig: Replace USE_OF with OF

USE_OF is used as intermediate Kconfig option by few arch's (ARM, MIPS,
Xtensa); in all these cases it implies setting option OF too. Replace the
only instance of USE_OF in clocksource with OF.
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/Kconfig

@@ -277,7 +277,7 @@ config CLKSRC_MIPS_GIC
 
 config CLKSRC_PXA
 	def_bool y if ARCH_PXA || ARCH_SA1100
-	select CLKSRC_OF if USE_OF
+	select CLKSRC_OF if OF
 	help
 	  This enables OST0 support available on PXA and SA-11x0
 	  platforms.