net: dsa: bcm_sf2: communicate integrated PHY revision to PHY driver

The integrated BCM7xxx PHY contains no useful revision information
in its MII_PHYSID2 bits 3:0, that information is instead contained in
the SWITCH_REG_PHY_REVISION register.

Read this register, store its value, and return it by implementing the
dsa_switch::get_phy_flags() callback accordingly. The register layout is
already matching what the BCM7xxx PHY driver is expecting to find.

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