iio: hi8435: add raw access

With current event-only driver, it is not possible for user space
application to know current senses if they don't change since
application starts.

Address that by adding raw access to channels.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>