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/imx8mq_evk_defconfig b/configs/imx8mq_evk_defconfig
index 6f38652..2dc5b6c 100644
--- a/configs/imx8mq_evk_defconfig
+++ b/configs/imx8mq_evk_defconfig
@@ -48,6 +48,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
+CONFIG_FEC_MXC=y
 CONFIG_PHY=y
 CONFIG_PHY_IMX8MQ_USB=y
 CONFIG_PINCTRL=y