net: bgmac: support Ethernet core on BCM53573 SoCs

BCM53573 is a new series of Broadcom's SoCs. It's based on ARM and can
be found in two packages (versions): BCM53573 and BCM47189. It shares
some code with the Northstar family, but also requires some new quirks.

First of all there can be up to 2 Ethernet cores on this SoC. If that is
the case, they are connected to two different switch ports allowing some
more complex/optimized setups. It seems the second unit doesn't come
fully configured and requires some IRQ quirk.

Other than that only the first core is connected to the PHY. For the
second one we have to register fixed PHY (similarly to the Northstar),
otherwise generic PHY driver would get some invalid info.

This has been successfully tested on Tenda AC9 (BCM47189B0).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>