Convert CONFIG_FEC_MXC to Kconfig

This converts the following to Kconfig:
	CONFIG_FEC_MXC

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index bcab3d6..b823a15 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -93,6 +93,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
+CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_DM_PMIC=y
 CONFIG_DM_REGULATOR=y