net: mvpp2: initialize the GoP

The patch adds GoP (group of ports) initialization functions. The mvpp2
driver was relying on the firmware/bootloader initialization; this patch
moves this setup to the mvpp2 driver.

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>