iio: ak8975: add definition structure per compass type

For each type of compass supported (AK8975 and AK8963),
add a definition structure for register masks, important registers,
raw data interpretation.
This change will make integrating new type of devices easier.

Remove i2c register cache. It is only used for one single register.
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>