iio: mxs-lradc: clarify supported devices

At the beginning the driver supported only i.MX28 SoC, but now the
whole MXS platform. So remove any confusing comments which apply
only to i.MX28.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>