dt-bindings: vendor-prefixes: Add apple prefix

This is different from the legacy AAPL prefix used on PPC, but
consensus is that we prefer `apple` for these new platforms.

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>