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/m53menlo_defconfig b/configs/m53menlo_defconfig
index c82589d..8eaaa83 100644
--- a/configs/m53menlo_defconfig
+++ b/configs/m53menlo_defconfig
@@ -84,6 +84,7 @@
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_RTC_M41T62=y
+CONFIG_MXC_UART=y
 CONFIG_SYSRESET=y
 CONFIG_SYSRESET_WATCHDOG=y
 CONFIG_USB=y