staging: iio: cdc: ad7152: Implement IIO_CHAN_INFO_SAMP_FREQ attribute

Attributes that were once privately defined become standard with time
and hence a special global define is used. Hence update driver ad7152 to use
IIO_CHAN_INFO_SAMP_FREQ which is a global define instead of
IIO_DEV_ATTR_SAMP_FREQ.
Move functionality from IIO_DEV_ATTR_SAMP_FREQ attribute into
IIO_CHAN_INFO_SAMP_FREQ to implement the sampling_frequency attribute.
Modify ad7152_read_raw() and ad7152_write_raw() to allow reading and
writing the element as well. Also add a lock in the driver's private
data.

Signed-off-by: sayli karnik <karniksayli1995@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>