clocksource: convert MXS timrotv2 to 32-bit down counting clocksource

Convert the MXS timrotv2 32-bit down counting clocksource to the
generic clocksource infrastructure.

Cc: Sascha Hauer <kernel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index cbc27fb..43f003a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -376,6 +376,7 @@
 	select GENERIC_CLOCKEVENTS
 	select ARCH_REQUIRE_GPIOLIB
 	select CLKDEV_LOOKUP
+	select CLKSRC_MMIO
 	help
 	  Support for Freescale MXS-based family of processors