net: mvpp2: initialize the XLG MAC when using a port

This adds a routine to initialize the XLG MAC at the port level when
using a port and the XAUI/10GKR interface mode. 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>