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/xpress_spl_defconfig b/configs/xpress_spl_defconfig
index d446736..33e5a77 100644
--- a/configs/xpress_spl_defconfig
+++ b/configs/xpress_spl_defconfig
@@ -47,6 +47,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_SMSC=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_OF_LIBFDT=y