imx: introduce CONFIG_GPT_TIMER

Introduce a new config symbol to select the i.MX
General Purpose Timer (GPT).

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/arch/arm/mach-imx/Makefile b/arch/arm/mach-imx/Makefile
index d77c10e..9322c1c 100644
--- a/arch/arm/mach-imx/Makefile
+++ b/arch/arm/mach-imx/Makefile
@@ -11,7 +11,8 @@
 obj-y	= iomux-v3.o
 endif
 ifeq ($(SOC),$(filter $(SOC),mx5 mx6))
-obj-y	+= timer.o cpu.o speed.o
+obj-y	+= cpu.o speed.o
+obj-$(CONFIG_GPT_TIMER) += timer.o
 obj-$(CONFIG_SYS_I2C_MXC) += i2c-mxv7.o
 endif
 ifeq ($(SOC),$(filter $(SOC),mx7 mx6 mxs))