mainboard/opencellular/elgon: Add mainboard support

Tested on Elgon EVT board and boots into GNU/Linux.

TODO:
* Add hard reset function for VBOOT.
* Add EC code
* Add SPI flash write protection

Change-Id: I9b809306cc48facbade5dc63846c4532b397e0b5
Signed-off-by: Philipp Deppenwiese <zaolin@das-labor.org>
Reviewed-on: https://review.coreboot.org/28024
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

Documentation/mainboard/index.md

@@ -18,6 +18,10 @@ This section contains documentation about coreboot on specific mainboards.
 
 - [GA-H61M-S2PV](gigabyte/ga-h61m-s2pv.md)
 
+## Open Cellular
+
+- [Elgon](opencellular/elgon.md)
+
 ## HP
 
 - [Compaq 8200 Elite SFF](hp/compaq_8200_sff.md)

Documentation/mainboard/opencellular/elgon.md

@@ -0,0 +1,72 @@
+# Elgon
+
+This page describes how to run coreboot on the [Elgon] compute board
+from [OpenCellular].
+
+## TODO
+
+* Add hard reset control
+
+## Flashing coreboot
+
+```eval_rst
++---------------------+------------+
+| Type                | Value      |
++=====================+============+
+| Socketed flash      | no         |
++---------------------+------------+
+| Model               | W25Q128    |
++---------------------+------------+
+| Size                | 16 MiB     |
++---------------------+------------+
+| In circuit flashing | yes        |
++---------------------+------------+
+| Package             | SOIC-8     |
++---------------------+------------+
+| Write protection    | No         |
++---------------------+------------+
+| Dual BIOS feature   | No         |
++---------------------+------------+
+| Internal flashing   | yes        |
++---------------------+------------+
+```
+
+### Internal programming
+
+The SPI flash can be accessed using [flashrom].
+
+### External programming
+
+The EVT board does have a pinheader to flash the SOIC-8 in circuit.
+Directly connecting a Pomona test-clip on the flash is also possible.
+
+TODO: pinout
+
+**Total board view of EVT**
+
+![][elgon1]
+
+[elgon1]: elgon1.png
+
+**Closeup view of SOIC-8 flash IC, programming pin header and
+USB serial connector of EVT**
+
+![][elgon2]
+
+[elgon2]: elgon2.png
+
+## Technology
+
+```eval_rst
++---------------+----------------------------------------+
+| SoC           | :doc:`../../soc/cavium/cn81xx/index`   |
++---------------+----------------------------------------+
+| CPU           | Cavium ARMv8-Quadcore `CN81XX`_        |
++---------------+----------------------------------------+
+
+.. _CN81XX: https://www.cavium.com/product-octeon-tx-cn80xx-81xx.html
+```
+
+[Elgon]: https://github.com/Telecominfraproject/OpenCellular
+[OpenCellular]: https://code.fb.com/connectivity/introducing-opencellular-an-open-source-wireless-access-platform/
+[flashrom]: https://flashrom.org/Flashrom