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/somlabs_visionsom_6ull_defconfig b/configs/somlabs_visionsom_6ull_defconfig
index f8c9e1f..091c5b2 100644
--- a/configs/somlabs_visionsom_6ull_defconfig
+++ b/configs/somlabs_visionsom_6ull_defconfig
@@ -45,6 +45,7 @@
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_STORAGE=y