boards: arm: particle*: add header nexus maps and feather labels

The Gen3 (formerly "mesh") Particle product line has a header that is
structurally related to the Adafruit Feather, and is generally
compatible with Featherwing shields.  Provide nexus maps for both the
native header layout, and for the subset feather header layout, and
add alias labels for the peripherals that would be referenced from
shield overlays.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>