arm: dts: armada-cp110-*dtsi: add xmdio nodes

Based on upstream-linux
See https://github.com/torvalds/linux/commit/f66b2aff.

However made the XSMI register window 0x16 (22) bytes per my reading
of the functional spec.  Similar commits in Marvels own repo bump it
to 0x200 (512) bytes but I did not see the reasoning for that.

https://github.com/MarvellEmbeddedProcessors/u-boot-marvell/commit/4d932b4

.

Also added device-name attributes to prevent ambiguity in the `mdio`
command.

Signed-off-by: Nevo Hed <nhed+github@starry.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>