iio: accel: add support for FXLS8962AF/FXLS8964AF accelerometers

Add basic support for NXP FXLS8962AF/FXLS8964AF Automotive
accelerometers.
It will allow setting up scale/gain and reading x,y,z
axis.

Datasheet: https://www.nxp.com/docs/en/data-sheet/FXLS8962AF.pdf
Datasheet: https://www.nxp.com/docs/en/data-sheet/FXLS8964AF.pdf
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>