iio: Provide iio_read_channel_processed_scale() API

iio: Provide iio_read_channel_processed_scale() API

Since the old iio_read_channel_processed() would
lose precision if we fall back to reading raw and
scaling, we introduce a new API that will pass in
a scale factor when reading a processed channel:
iio_read_channel_processed_scale().

Refactor iio_read_channel_processed() as a special
case with scale factor 1.

Cc: Peter Rosin <peda@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Link: https://lore.kernel.org/linux-iio/20201224011607.1059534-1-linus.walleij@linaro.org/
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210308100219.2732156-1-linus.walleij@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>