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/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index a412638..32eae00 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -83,6 +83,7 @@
 CONFIG_MII=y
 CONFIG_DM_PMIC=y
 CONFIG_DM_REGULATOR=y
+CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_MXC_SPI=y