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>