iio: mxs-lradc: add write_raw function to modify scale

Added write_raw function to manipulate the optional divider_by_two
through the scaling attribute out of the available scales.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>