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/gwventana_emmc_defconfig b/configs/gwventana_emmc_defconfig
index 9e10bdf..fc2504e 100644
--- a/configs/gwventana_emmc_defconfig
+++ b/configs/gwventana_emmc_defconfig
@@ -76,6 +76,7 @@
 CONFIG_MII=y
 CONFIG_PCI=y
 CONFIG_DM_SERIAL=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y