mb/up/squared: Add mainboard

Works:
- bootblock, romstage, ramstage
- Serial console UART0, UART1
- SPI flash console
- iGPU init with libgfxinit
- LAN1, LAN2
- USB2, USB3
- HDMI, DisplayPort
- eMMC
- flashing with flashrom externally

WIP:
- Documentation
- VGA
    For some reason Seabios can not find the CBFS region
    and therefore it can't load seavgabios, but generally
    it is working as soon as Linux is booted.
- ACPI

Works not:
- Devices needs proper configuration
- Seabios can't find CBFS region

Untested:
- GPIO pin header
- 60 pin EXHAT
- Camera interface
  - MIPI-CSI2 2-lane (2MP)
  - MIPI-CSI2 4-lane (8MP)
- SATA3
- USB3 OTG
- embedded DisplayPort
- M.2 slot
- mini PCIe
- flashing with flashrom internally using Linux

Change-Id: Ia913534ec176fc600fcd4ce3af335ebe682b0ed4
Signed-off-by: Felix Singer <felix.singer@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31378
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/mainboard/index.md

@@ -82,3 +82,7 @@ The boards in this section are not real mainboards, but emulators.
 ## Supermicro
 
 - [X10SLM+-F](supermicro/x10slm-f.md)
+
+## UP
+
+- [Squared](up/squared/index.md)