Merge branch 'Add-DSA-yaml-binding'

Kurt Kanzenbach says:

====================
Add DSA yaml binding

as discussed [1] [2] it makes sense to add a DSA yaml binding. This is the
second version and contains now two ways of specifying the switch ports: Either
by "ports" or by "ethernet-ports". That is why the third patch also adjusts the
DSA core for it.

Tested in combination with the hellcreek.yaml file.

Changes since v1:

 * Use select to not match unrelated switches
 * Allow ethernet-port(s)
 * List ethernet-controller properties
 * Include better description
 * Let dsa.txt refer to dsa.yaml

Thanks,
Kurt

[1] - https://lkml.kernel.org/netdev/449f0a03-a91d-ae82-b31f-59dfd1457ec5@gmail.com/
[2] - https://lkml.kernel.org/netdev/20200710090618.28945-1-kurt@linutronix.de/
====================

Signed-off-by: David S. Miller <davem@davemloft.net>