net: ethernet: bgmac: move BCMA MDIO Phy code into a separate file

Move the BCMA MDIO phy into a separate file, as it is very tightly
coupled with the BCMA bus.  This will help with the upcoming BCMA
removal from the bgmac driver.  Optimally, this should be moved into
phy drivers, but it is too tightly coupled with the bgmac driver to
effectively move it without more changes to the driver.

Note: the phy_reset was intentionally removed, as the mdio phy subsystem
automatically resets the phy if a reset function pointer is present.  In
addition to the moving of the driver, this reset function is added.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>