net: mvpp2: enable basic 10G support

On GOP port 0 two MAC modes are available: GMAC and XLG. The XLG MAC is
used for 10G connectivity. This patch adds a basic 10G support by
allowing to use the XLG MAC on port 0 and by reworking the
port_enable/disable functions so that the XLG MAC is configured when
using 10G.

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