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/mx51evk_defconfig b/configs/mx51evk_defconfig
index 5dfa3be..22126cf 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -28,6 +28,7 @@
 CONFIG_FSL_ESDHC_IMX=y
 CONFIG_MTD=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y
 CONFIG_USB=y