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/nitrogen6s_defconfig b/configs/nitrogen6s_defconfig
index e320bec..66db493 100644
--- a/configs/nitrogen6s_defconfig
+++ b/configs/nitrogen6s_defconfig
@@ -66,6 +66,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
+CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_MXC_UART=y
 CONFIG_SPI=y