netdev/phy: Add driver for Broadcom BCM87XX 10G Ethernet PHYs

Add a driver for BCM8706 and BCM8727 devices.  These are a 10Gig PHYs
which use MII_ADDR_C45 addressing.  They are always 10G full duplex, so
there is no autonegotiation.  All we do is report link state and send
interrupts when it changes.

If the PHY has a device tree of_node associated with it, the
"broadcom,c45-reg-init" property is used to supply register
initialization values when config_init() is called.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>