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/warp_defconfig b/configs/warp_defconfig
index e981f51..a9def8a 100644
--- a/configs/warp_defconfig
+++ b/configs/warp_defconfig
@@ -33,6 +33,7 @@
 CONFIG_DFU_MMC=y
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_FSL_USDHC=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_GADGET=y