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/sheevaplug_defconfig b/configs/sheevaplug_defconfig
index d921666..25a8e53 100644
--- a/configs/sheevaplug_defconfig
+++ b/configs/sheevaplug_defconfig
@@ -27,6 +27,8 @@
 CONFIG_ISO_PARTITION=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_MVSATA_IDE=y
+CONFIG_NETDEVICES=y
+CONFIG_MVGBE=y
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y