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/zmx25_defconfig b/configs/zmx25_defconfig
index 24eb20b..eb6c0ab 100644
--- a/configs/zmx25_defconfig
+++ b/configs/zmx25_defconfig
@@ -31,6 +31,7 @@
 CONFIG_SYS_FLASH_USE_BUFFER_WRITE=y
 CONFIG_SYS_FLASH_CFI=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y