net: mvgbe: fallback phy-mode to GMII

Some existing device trees don't specify a phy-mode so fallback to GMII
when a phy-mode is not provided.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>