Staging: iio: accel: adis16201: Group register definitions

Group register definitions with its register field bits to improve
readability and easy identification. A small comment is also added to
denote the purpose/functionality of the grouped register definitions.

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>