net: phy: Prepare for moving Omega out of bcm7xxx

The Omega PHY entry was added to bcm7xxx.c out of convenience and this
breaks the one driver per product line paradigm that was applied up
until now. Since the AFE initialization is shared between Omega and
BCM7xxx move the relevant functions to bcm-phy-lib.[ch]. No functional
changes introduced.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>