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/sksimx6_defconfig b/configs/sksimx6_defconfig
index 304be45..19870b2 100644
--- a/configs/sksimx6_defconfig
+++ b/configs/sksimx6_defconfig
@@ -45,5 +45,6 @@
 CONFIG_PHY_MICREL_KSZ8XXX=y
 CONFIG_FEC_MXC=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y
 CONFIG_OF_LIBFDT=y