mb/system76/gaze16: Add System76 Gazelle 16

https://tech-docs.system76.com/models/gaze16/README.html

The gaze16 comes in 3 variants due to differences in the discrete GPU
and network controller used.

- NVIDIA RTX 3050, using Realtek Ethernet controller
- NVIDIA RTX 3060, using Realtek Ethernet controller
- NVIDIA RTX 3060, using onboard Intel I219-V Ethernet controller

Tested on the 3050 variant.
Tested with TianoCore (UefiPayloadPkg).

Working:

- PS/2 keyboard, touchpad
- Both DIMM slots
- M.2 NVMe SSD
- M.2 SATA SSD
- 2.5" SSD
- All USB ports
- SD card reader
- Webcam
- Ethernet
- WiFi/Bluetooth
- Integrated graphics using Intel GOP driver
- HDMI output
- Internal microphone
- Internal speakers
- Combined headphone + mic 3.5mm audio*
- 3.5mm microphone input*
- S3 suspend/resume
- Booting to Pop!_OS Linux 21.04 and Windows 10 20H2
- Flashing with flashrom

Not working:

- Discrete/Hybrid graphics
- Mini DisplayPort output (requires NVIDIA GPU)
- 3.5mm audio input/output detection on Windows

Change-Id: Ifb90f9b73a10abf53a21738e2c466d539df9a37c
Signed-off-by: Jeremy Soller <jeremy@system76.com>
Signed-off-by: Tim Crawford <tcrawford@system76.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56956
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/mainboard/index.md

@@ -188,6 +188,7 @@ The boards in this section are not real mainboards, but emulators.
 - [Galago Pro 4](system76/galp4.md)
 - [Galago Pro 5](system76/galp5.md)
 - [Gazelle 15](system76/gaze15.md)
+- [Gazelle 16](system76/gaze16.md)
 - [Lemur Pro 9](system76/lemp9.md)
 - [Lemur Pro 10](system76/lemp10.md)
 - [Oryx Pro 5](system76/oryp5.md)