imx6: add support for the intel gbit driver on GW5520

The Gateworks GW5520 board uses 2x intel gigabit mac's, instead of the
internal i.MX6 FEC.

Add support for these.

CC: Scott V. Kamp <outbackdingo@gmail.com>
CC: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>

target/linux/imx6/config-4.14

@@ -172,6 +172,7 @@ CONFIG_DMA_OF=y
 # CONFIG_DMA_VIRT_OPS is not set
 # CONFIG_DRM_LIB_RANDOM is not set
 CONFIG_DTC=y
+CONFIG_E1000E=y
 CONFIG_EDAC_ATOMIC_SCRUB=y
 CONFIG_EDAC_SUPPORT=y
 CONFIG_ENCRYPTED_KEYS=y