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/riotboard_defconfig b/configs/riotboard_defconfig
index f02274e..4d8cd54 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -32,6 +32,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y
 CONFIG_DM_THERMAL=y