dt-bindings: magnetometer: ak8974: Add Alps hscdtd008a

Add binding documentation for the Alps HSCDTD008A, which is similar
to the ak8974.

Signed-off-by: Nick Reitemeyer <nick.reitemeyer@web.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>