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/tbs2910_defconfig b/configs/tbs2910_defconfig
index 62d606b..61b5a8b 100644
--- a/configs/tbs2910_defconfig
+++ b/configs/tbs2910_defconfig
@@ -79,6 +79,7 @@
 CONFIG_PINCTRL_IMX6=y
 CONFIG_DM_RTC=y
 CONFIG_RTC_DS1307=y
+CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y
 CONFIG_DM_USB=y