ath79: rename Ubiquiti PowerBeam M (XW) to PowerBeam M5 (XW)

ath79: rename Ubiquiti PowerBeam M (XW) to PowerBeam M5 (XW)

The commit [1] added support for Ubiquiti PowerBeam M (XW), tested
on the PBE-M5-400. But, it turns out the PBE-M2-400 has a different
ethernet configuration, so make the support specific to the m5 version
in anticipation of adding specific support for the m2 in a separate
commit.

[1] 12eb5b2384a6 ("ath79: add support for Ubiquiti PowerBeam M (XW)")

Signed-off-by: Russell Senior <russell@personaltelco.net>
[fix model name in DTS, format commit reference in commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>