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/nitrogen6s1g_defconfig b/configs/nitrogen6s1g_defconfig
index 237d7a8..4adbbc4 100644
--- a/configs/nitrogen6s1g_defconfig
+++ b/configs/nitrogen6s1g_defconfig
@@ -59,6 +59,7 @@
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_MXC_SPI=y