ath79: DTS style improvement for Ubiquiti XM and BZ devices

ath79: DTS style improvement for Ubiquiti XM and BZ devices

This applies minor style improvements and removes commented pll
clock adjustments from ubnt_xm DTSI. The latter were introduced
(already commented out) when adding ath79 target and have never
been touched since then. For Unifi (BZ board), similar clock
adjustments are employed and used.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>