net: phy: add Broadcom BCM54140 support

The Broadcom BCM54140 is a Quad SGMII/QSGMII Copper/Fiber Gigabit
Ethernet transceiver.

This also adds support for tunables to set and get downshift and
energy detect auto power-down.

The PHY has four ports and each port has its own PHY address.
There are per-port registers as well as global registers.
Unfortunately, the global registers can only be accessed by reading
and writing from/to the PHY address of the first port. Further,
there is no way to find out what port you actually are by just
reading the per-port registers. We therefore, have to scan the
bus on the PHY probe to determine the port and thus what address
we need to access the global registers.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>