Documentation/bindings: phy: document the Marvell comphy driver

The Marvell Armada 7K/8K SoCs contains an hardware block called COMPHY
that provides a number of shared PHYs used by various interfaces in the
SoC: network, SATA, PCIe, etc. This Device Tree binding allows to
describe this COMPHY hardware block.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>