imx: add directory drivers/mxc for i.MX specific drivers

Signed-off-by: Eric Miao <eric.miao@linaro.org>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 95b9e7e..00c6526 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -130,4 +130,5 @@
 
 source "drivers/virt/Kconfig"
 
+source "drivers/mxc/Kconfig"
 endmenu