net: bgmac: use PHY subsystem for initializing PHY

This adds support for using bgmac with PHYs supported by standalone PHY
drivers. Having any PHY initialization in bgmac is hacky and shouldn't
be extended but rather removed if anyone has hardware to test it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>