net: phy: bcm7xxx: request and manage GPHY clock

The internal Gigabit PHY on Broadcom STB chips has a digital clock which
drives its MDIO interface among other things, the driver now requests
and manage that clock during .probe() and .remove() accordingly.

Because the PHY driver can be probed with the clocks turned off we need
to apply the dummy BMSR workaround during the driver probe function to
ensure subsequent MDIO read or write towards the PHY will succeed.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>