net: mvgbe: convert to DM

Add driver model support to the mvgbe driver. As a temporary measure
both DM and non-DM uses are supported. Once all the users have been
converted the non-DM support can be dropped.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Michael Walle <michael@walle.cc>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>