ARM: dt-bindings: Add vendor prefix for Facebook

Initial introduction of Facebook TiogaPass family equipped with
Aspeed 2500 BMC SoC. TiogaPass is a x86 server development kit
with a ASPEED ast2500 BMC manufactured by Facebook.

This adds an entry of Facebook in Documentation for vendor prefix

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>