net: phy: Move Omega PHY entry to Cygnus PHY driver

Cygnus and Omega are part of the same business unit and product line, it
makes sense to group PHY entries by products such that a platform can
select only the drivers that it needs. Bring all the functionality that
the BCM7XXX_28NM_GPHY() macro hides for us and remove the Omega PHY
entry from bcm7xxx.c.

As an added bonus, we now have a proper mdio_device_id entry to permit
auto-loading.

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>