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/pico-imx6_defconfig b/configs/pico-imx6_defconfig
index 2bd2203..5390746 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -69,6 +69,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
+CONFIG_FEC_MXC=y
 CONFIG_RGMII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y