arm64: dts: librem5-r3: Mark buck3 as always on

Commit 66d3f246 removed always-on
marking from GPU regulator, which is great for power saving - however,
it introduces additional i2c0 traffic which can be deadly for devices
from the Dogwood batch.

To workaround the i2c0 shutdown issue on Dogwood, this commit marks
buck3 as always-on again - but only for Dogwood (r3).
2 jobs for dogwood-buck3
in 62 minutes and 9 seconds, using 0.0 compute credits, and was queued for 2 seconds