iio: mxs-lradc: simplify TS registration

This patch simplifies the TS registration of mxs-lradc by
using devm_input_allocate_device.

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