net: mvpp2: initialize the GMAC when using a port

This adds a routine to initialize the GMAC at the port level when using
a port. This wasn't done until this commit, and the mvpp2 driver was
relying on the bootloader/firmware initialization. This doesn't mean
everything is configured in the mvpp2 driver now, but it helps reducing
the gap.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>