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/apalis_imx6_defconfig b/configs/apalis_imx6_defconfig
index e162f0a..2fce888 100644
--- a/configs/apalis_imx6_defconfig
+++ b/configs/apalis_imx6_defconfig
@@ -84,6 +84,7 @@
 CONFIG_DM_REGULATOR_PFUZE100=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_SCSI=y
+CONFIG_MXC_UART=y
 CONFIG_IMX_THERMAL=y
 CONFIG_USB=y
 CONFIG_DM_USB=y