net: phy: extend fixed driver with fixed_phy_register()

The existing fixed_phy_add() function has several drawbacks that
prevents it from being used as is for OF-based declaration of fixed
PHYs:

 * The address of the PHY on the fake bus needs to be passed, while a
   dynamic allocation is desired.

 * Since the phy_device instantiation is post-poned until the next
   mdiobus scan, there is no way to associate the fixed PHY with its
   OF node, which later prevents of_phy_connect() from finding this
   fixed PHY from a given OF node.

To solve this, this commit introduces fixed_phy_register(), which will
allocate an available PHY address, add the PHY using fixed_phy_add()
and instantiate the phy_device structure associated with the provided
OF node.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>