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/platinum_picon_defconfig b/configs/platinum_picon_defconfig
index f700233..0836932 100644
--- a/configs/platinum_picon_defconfig
+++ b/configs/platinum_picon_defconfig
@@ -59,6 +59,7 @@
 CONFIG_NAND_MXS=y
 CONFIG_PHYLIB=y
 CONFIG_MII=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_OF_LIBFDT=y