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/vf610twr_nand_defconfig b/configs/vf610twr_nand_defconfig
index e320477..44a0028 100644
--- a/configs/vf610twr_nand_defconfig
+++ b/configs/vf610twr_nand_defconfig
@@ -51,6 +51,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ8XXX=y
+CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_DM_SERIAL=y
 CONFIG_FSL_LPUART=y