net: bcmgenet: Add support for 7712 16nm internal EPHY

The 16nm internal EPHY that is present in 7712 is actually a 16nm
Gigabit PHY which has been forced to operate in 10/100 mode. Its
controls are therefore via the EXT_GPHY_CTRL registers and not via the
EXT_EPHY_CTRL which are used for all GENETv5 adapters. Add a match on
the 7712 compatible string to allow that differentiation to happen.

On previous GENETv4 chips the EXT_CFG_IDDQ_GLOBAL_PWR bit was cleared by
default, but this is not the case with this chip, so we need to make
sure we clear it to power on the EPHY.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>