devicetree: Add Fujitsu Ltd. vendor prefix

The vendor prefix for Fujitsu is used in the tree, but it's
still missing from the documentation, so add it. Fujitsu Ltd.
is a japanese ICT company, http://www.fujitsu.com

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>