net: dsa: b53: Add BCM7445 quirk

The Broadcom BCM7445 STB chip has an issued in its revision D0 which was
previously worked around in drivers/net/dsa/bcm_sf2.c where we may
end-up double programming the integrated BCM7445 switch (bcm_sf2) and an
external Broadcom switch such as BCM53125, since these are mostly
register compatible.

Add a small quirk which just defers probing until we are sitting on the
slave DSA MDIO bus, which will allow us to intercept reads/writes and
funnel them through the SF2 internal MDIO master (which happens to
disconnect its pseudo PHY).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>