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/mx6cuboxi_defconfig b/configs/mx6cuboxi_defconfig
index cd4d4da..061d1f6 100644
--- a/configs/mx6cuboxi_defconfig
+++ b/configs/mx6cuboxi_defconfig
@@ -61,6 +61,7 @@
 CONFIG_PINCTRL_IMX6=y
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
+CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y
 CONFIG_DM_USB=y