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/warp7_defconfig b/configs/warp7_defconfig
index 2d10529..ace71a7 100644
--- a/configs/warp7_defconfig
+++ b/configs/warp7_defconfig
@@ -55,6 +55,7 @@
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_SPECIFY_CONSOLE_INDEX=y
 CONFIG_DM_SERIAL=y
+CONFIG_MXC_UART=y
 CONFIG_OPTEE=y
 CONFIG_USB=y
 CONFIG_DM_USB=y