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/mx5/Kconfig b/arch/arm/mach-imx/mx5/Kconfig
index 250194b..3ce6bcf 100644
--- a/arch/arm/mach-imx/mx5/Kconfig
+++ b/arch/arm/mach-imx/mx5/Kconfig
@@ -1,6 +1,7 @@
 if ARCH_MX5
 
 config MX5
+	select GPT_TIMER
 	bool
 	default y