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/cl-som-imx7_defconfig b/configs/cl-som-imx7_defconfig
index bb96afc..faad487 100644
--- a/configs/cl-som-imx7_defconfig
+++ b/configs/cl-som-imx7_defconfig
@@ -80,6 +80,7 @@
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
+CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_POWER_LEGACY=y
 CONFIG_DM_REGULATOR=y