dt-bindings: vendor-prefixes: add iMX6 Rex Project

Document vendor prefix for iMX6 Rex Project.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200926162811.5335-2-krzk@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>