net: dsa: allow switch drivers to specify phy_device::dev_flags

Some switch drivers (e.g: bcm_sf2) may have to communicate specific
workarounds or flags towards the PHY device driver. Allow switches
driver to be delegated that task by introducing a get_phy_flags()
callback which will do just that.

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