iio: gyro: adxrs290: Add debugfs register access support

Extend support to read/write byte data from/to the device using
debugfs iio interface.

Signed-off-by: Nishant Malpani <nish.malpani25@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200910180450.29696-4-nish.malpani25@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>