arm64: dts: fsl: librem5: Add a device tree for the Librem5 devkit

This is for the development kit board for the Librem 5. The current level
of support yields a working console and is able to boot userspace from
the network or eMMC.

Additional subsystems that are active :

- Both USB ports
- SD card socket
- WiFi usdhc
- WWAN modem
- GNSS
- GPIO keys
- LEDs
- gyro
- magnetometer
- touchscreen
- pwm
- backlight
- haptic motor

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>