Documentation: ACPI: DSD: Document MDIO PHY

Introduce a mechanism based on generic ACPI _DSD device properties
definition [1] to get PHYs registered on a MDIO bus and provide them to
be connected to MAC.

[1] http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf

Describe properties "phy-handle" and "phy-mode".

Signed-off-by: Calvin Johnson <calvin.johnson@oss.nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Acked-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>