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/mx6qarm2_lpddr2_defconfig b/configs/mx6qarm2_lpddr2_defconfig
index 86f1224..6a31946 100644
--- a/configs/mx6qarm2_lpddr2_defconfig
+++ b/configs/mx6qarm2_lpddr2_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_FSL_USDHC=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_HOST_ETHER=y