Convert CONFIG_MXC_UART to Kconfig

This converts the following to Kconfig:
   CONFIG_MXC_UART

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/mx53evk_defconfig b/configs/mx53evk_defconfig
index 9063f72..b36d916 100644
--- a/configs/mx53evk_defconfig
+++ b/configs/mx53evk_defconfig
@@ -20,4 +20,5 @@
 CONFIG_FSL_ESDHC_IMX=y
 CONFIG_MTD=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_OF_LIBFDT=y