Add Librem5r2 and r3

This adds the support for the Librem 5's Birch/Chestnut (r2) and Dogwood revisions.

The device trees are not yet merged mainline but it went through several revisions already https://lore.kernel.org/linux-arm-kernel/20200731082725.21878-1-martin.kepplinger@puri.sm/ so i expect the model names to remain stable.