net: add Kconfig for MVGBE

Add Kconfig for MVGBE and update boards to select this.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/configs/mgcoge3un_defconfig b/configs/mgcoge3un_defconfig
index 918cc61..abf9cdc 100644
--- a/configs/mgcoge3un_defconfig
+++ b/configs/mgcoge3un_defconfig
@@ -31,6 +31,8 @@
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_NETDEVICES=y
+CONFIG_MVGBE=y
 CONFIG_SYS_NS16550=y
 CONFIG_SPI=y
 CONFIG_KIRKWOOD_SPI=y