arm64: dts: Add a device tree for the Librem 5 phone

Add a devicetree description for the Librem 5 phone. 4 hardware revisions
have been available. Some revisions include changes that need different
software to be run. So far, r3 ("Dogwood") is one such example, see:

	"Aspen"		r0	not supported (very few devices exist)
	"Birch"		r1	supported by r2
	"Chestnut"	r2	added by this patch
	"Dogwood"	r3	added by this patch
	"Evergreen"	r4	tba / most likely supported by r3

See https://puri.sm/products/librem-5/ for more information.

This boots to a working console with working WWAN modem, wifi usdhc,
IMU sensor device, proximity sensor, haptic motor, gpio keys, GNSS and LEDs.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com> (for the audio part)
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>