net: phy: broadcom: add helper for PHY revision and patch level

The Broadcom BCM7xxx internal PHYs do not contain any useful revision
information in the low 4-bits of their MII_PHYSID2 (MII register 3)
which could allow us to properly identify them.

As a result, we need the actual hardware block integrating these PHYs:
GENET or the SF2 switch to tell us what revision they are built with. To
assist with that, add two helper macros for fetching the the PHY
revision and patch level from the struct phy_device::dev_flags.

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