purism-librem5: change default device tree

Based on some new (to me...) info from Purism's Sebastian K, there are apparently Evergreen devices that return different values for board rev, so we shouldn't assume only r4 are Evergreen, and default to the r2 DT.

But many earlier devices (Birch, Chestnut, Dogwood) may or may not have the board rev set either. Who knows! Defaulting to the Evergreen DT on these devices is bad too, but there weren't many of those shipped (I guess?), at least not compared to the number of Evergreens that aren't properly identifiable in u-boot... so this using this DT by default is the best of the two bad options we have.

This comment thread has more info: #1643 (closed) (comment 1147248594)

fixes #1643 (closed)