dt-bindings: vendor-prefixes: Add Xiphera vendor prefix

Xiphera is an FPGA-based cryptographic solutions provider based in
Finland.
Website of the company: https://xiphera.com/



Signed-off-by: Atte Tommiska <atte.tommiska@xiphera.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>