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: Ye.Li <B37916@freescale.com>
diff --git a/include/configs/mx6_common.h b/include/configs/mx6_common.h
index 135a3f5..e0528ce 100644
--- a/include/configs/mx6_common.h
+++ b/include/configs/mx6_common.h
@@ -29,5 +29,6 @@
 #endif
 
 #define CONFIG_MP
+#define CONFIG_MXC_GPT_HCLK
 
 #endif