Staging/iio/adc/touchscreen/MXS: distinguish i.MX23's and i.MX28's LRADC

The LRADC units in i.MX23 and i.MX28 differ and we need to distinguish both
SoC variants in order to make the touchscreen work on i.MX23

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Tested-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

CC: linux-arm-kernel@lists.infradead.org
CC: linux-input@vger.kernel.org
CC: devel@driverdev.osuosl.org
CC: Fabio Estevam <fabio.estevam@freescale.com>
CC: Jonathan Cameron <jic23@cam.ac.uk>