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/usbarmory_defconfig b/configs/usbarmory_defconfig
index 0564bdc..a8bc0e6 100644
--- a/configs/usbarmory_defconfig
+++ b/configs/usbarmory_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_FSL_ESDHC_IMX=y
 CONFIG_MTD=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_MX5=y
 CONFIG_OF_LIBFDT=y