Staging: iio: meter: ade7758_core: implement IIO_CHAN_INFO_SAMP_FREQ

This driver predates the availability of IIO_CHAN_INFO_SAMP_FREQ attribute
wherein usage has some advantages like it can be accessed by in-kernel
consumers as well as reduces the code size.

Hence moving functionality from IIO_DEV_ATTR_SAMP_FREQ attribute into
IIO_CHAN_INFO_SAMP_FREQ handlers. Also Adding ade7758_read_raw() and
ade7758_write_raw() to allow reading/writing the element as well.

Signed-off-by: Sandhya Bankar <bankarsandhya512@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>