ARM: imx: move timer driver into drivers/clocksource

After the cleanup on imx timer driver, now it's ready to be moved into
drivers/clocksource/.  Let's do it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>

arch/arm/mach-imx/Kconfig

@@ -2,7 +2,7 @@ menuconfig ARCH_MXC
 	bool "Freescale i.MX family" if ARCH_MULTI_V4_V5 || ARCH_MULTI_V6_V7
 	select ARCH_REQUIRE_GPIOLIB
 	select ARM_CPU_SUSPEND if PM
-	select CLKSRC_MMIO
+	select CLKSRC_IMX_GPT
 	select GENERIC_IRQ_CHIP
 	select PINCTRL
 	select PM_OPP if PM