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>

Documentation/devicetree/bindings/iio/magnetometer/ak8974.txt

@@ -2,7 +2,9 @@
 
 Required properties:
 
-- compatible : should be "asahi-kasei,ak8974"
+- compatible:
+    * "asahi-kasei,ak8974"
+    * "alps,hscdtd008a"
 - reg : the I2C address of the magnetometer
 
 Optional properties: