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_phanbell_defconfig b/configs/imx8mq_phanbell_defconfig
index e1b60f7..062d2a4 100644
--- a/configs/imx8mq_phanbell_defconfig
+++ b/configs/imx8mq_phanbell_defconfig
@@ -52,6 +52,7 @@
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_FSL_USDHC=y
 CONFIG_DM_ETH=y
+CONFIG_FEC_MXC=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX8M=y
 CONFIG_SPL_POWER_LEGACY=y