imx: mx6: Enable high frequency clock source for GPT

Set the CONFIG_MXC_GPT_HCLK configuration in mx6_common.h, so that
24Mhz OSC clock source will be selected for GPT on all MX6 platforms.
Signed-off-by: NYe.Li <B37916@freescale.com>

include/configs/mx6_common.h

@@ -29,5 +29,6 @@
 #endif
 
 #define CONFIG_MP
+#define CONFIG_MXC_GPT_HCLK
 
 #endif