dt-bindings: arm: fsl: Add the librem 5 phone

Add entries for the imx8mq based Librem 5 phone. The "Birch" and
"Chestnut" hardware revisions are supported by r2. The "Dogwood"
revision by r3.
See https://puri.sm/products/librem-5/ and https://developer.puri.sm/Librem5/


for the schematics and more information.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>