Documentation: Add cavium SoC and mainboard

* Add documentation for CN81XX SoC
* Add documentation for CN81XX EVB SFF mainboard
* Add documentation for BDK
* Add documentation for BOOTROM and BOOTBLOCK behaviour
* Alphabetically sort vendors

Change-Id: Ibfcd42788e31f684baed658dc3c4dfe1b8e4f354
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/27150
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

Documentation/vendorcode/cavium/bdk.md

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+# Cavium's BDK
+
+## BDK
+A part of Cavium's BDK can be found in '''src/vendorcode/cavium/bdk'''.
+It does the **DRAM init** in romstage and the **PCIe**, **QLM**, **SLI**,
+**PHY**, **BGX**, **SATA** init in ramstage.
+
+## Devicetree
+The BDK does use it's own devicetree, as coreboot's devicetree isn't
+compatible. The devicetree stores key-value pairs (see **bdk-devicetree.h**
+for implementation details), where the key and the value are stored as strings.
+
+The key-value pairs must be advertised in romstage and ramstage using the
+'''bdk_config_set_fdt()''' method.
+
+The tool '''util/cavium/devicetree_convert.py''' can be used to convert a
+devicetree to a key-value array.
+
+## Modifications
+
+* The BDK has been modified to compile under coreboot's toolchain.
+* Removed FDT devicetree support.
+* Dropped files that aren't required for SoC bringup
+* Added Kconfig values for verbose console output
+
+## Debugging
+
+You can enable verbose console output in *menuconfig*:
+
+Go to **Chipset**, **BDK** and enable one or multiple stages.

Documentation/vendorcode/cavium/index.md

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+# Cavium vendorcode-specific documentation
+
+This section contains documentation about coreboot on Cavium specific
+vendorcode.
+
+## Sections
+
+- [BDK](bdk.md)

Documentation/vendorcode/index.md

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+# Vendorcode-specific documentation
+
+This section contains documentation about coreboot on specific vendorcode.
+
+## Vendor
+
+- [Cavium](cavium/index.md)