iio: mxs-lradc: change the realbits to 12

The LRADC virtual channels have an 18 bit field to store the sum of up
to 2^5 accumulated samples. The read_raw function however only operates
over a single sample (12 bit resolution).
In order to use this field for scaling operations, we need it to be the
exact resolution value of the LRADC.
Besides, the driver was using an 18 bit mask (LRADC_CH_VALUE_MASK) to
report touch coordinates to userland. A 12 bit mask should be used instead
or else the touch libraries will expect a coordinates range between 0
and 0x3ffff (18 bits), instead of between 0 and 0xfff (12 bits).

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