dt-bindings: vendor-prefixes: add DFI

Document binding for DFI Inc. company (https://www.dfi.com).

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200926162811.5335-1-krzk@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>