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/ts4800_defconfig b/configs/ts4800_defconfig
index 810f0d8..1c61412 100644
--- a/configs/ts4800_defconfig
+++ b/configs/ts4800_defconfig
@@ -23,6 +23,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_SMSC=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y
 CONFIG_OF_LIBFDT=y