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/colibri_imx7_defconfig b/configs/colibri_imx7_defconfig
index 3c49d70..425a3b1 100644
--- a/configs/colibri_imx7_defconfig
+++ b/configs/colibri_imx7_defconfig
@@ -77,6 +77,7 @@
 CONFIG_PINCTRL_IMX7=y
 CONFIG_DM_PMIC=y
 CONFIG_PMIC_RN5T567=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_EHCI_HCD=y