iio: accel: bma220: split original driver

iio: accel: bma220: split original driver

In preparation for the i2c module, move the original code into multiple
source files without any other functional change.

Create the additional bma220_core module which currently is not
providing an abstracted bus type (this will change with the regmap
patch).
Fix a few includes in the context of this patch.

Signed-off-by: Petre Rodan <petre.rodan@subdimension.ro>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>