dt-bindings: vendor-prefixes: add Embest RIoT

Document vendor prefix for Embest RIoT.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200926162811.5335-3-krzk@kernel.org


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