Documentation: ACPI: DSD: describe additional MAC configuration

Document additional MAC configuration modes which can be processed
by the existing fwnode_ phylink helpers:

* "managed" standard ACPI _DSD property [1]
* "fixed-link" data-only subnode linked in the _DSD package via
  generic mechanism of the hierarchical data extension [2]

[1] https://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf
[2] https://github.com/UEFI/DSD-Guide/blob/main/dsd-guide.pdf



Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>