iio: Documentation: Add missing documentation for power attribute

Commit c43a102e ("iio: ina2xx: add support for TI INA2xx Power Monitors")
introduced the in_powerY_raw attribute, but omitted the corresponding
documentation.

The description is correct for the INA2xx and the MAX9611 IIO drivers.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>