dt-bindings: vendor-prefixes: add second Synopsys prefix

There are few bindings with compatibles starting with a "synopsys"
prefix.  Document it as deprecated, to be sure no new usages will
appear.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220221082228.34407-2-krzysztof.kozlowski@canonical.com