net: rename: kirkwood_egiga as mvgbe

Rename kirkwood_egiga.* to mvgbe.* and adjust makefile
and #include accordingly.

Signed-off-by: Albert Aribaud <albert.aribaud@free.fr>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>