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/index.md

@@ -0,0 +1,8 @@
+# Cavium vendorcode-specific documentation
+
+This section contains documentation about coreboot on Cavium specific
+vendorcode.
+
+## Sections
+
+- [BDK](bdk.md)