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/mx6ull_14x14_evk_plugin_defconfig b/configs/mx6ull_14x14_evk_plugin_defconfig
index e6c31e6..e96402d 100644
--- a/configs/mx6ull_14x14_evk_plugin_defconfig
+++ b/configs/mx6ull_14x14_evk_plugin_defconfig
@@ -50,6 +50,7 @@
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y
 CONFIG_DM_REGULATOR=y
+CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_FSL_QSPI=y