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/mx53ard_defconfig b/configs/mx53ard_defconfig
index e021df2..ab1cd93 100644
--- a/configs/mx53ard_defconfig
+++ b/configs/mx53ard_defconfig
@@ -26,4 +26,5 @@
 CONFIG_MII=y
 CONFIG_SMC911X=y
 CONFIG_SMC911X_BASE=0xF4000000
+CONFIG_MXC_UART=y
 CONFIG_OF_LIBFDT=y