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/imx8mq_evk_defconfig b/configs/imx8mq_evk_defconfig
index 203f2e8..6ff02e7 100644
--- a/configs/imx8mq_evk_defconfig
+++ b/configs/imx8mq_evk_defconfig
@@ -57,4 +57,5 @@
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_DM_RESET=y
+CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y