iio: mxs: Add MX23 support into the IIO driver

This patch adds support for i.MX23 into the LRADC driver. The LRADC
block on MX23 is not much different from the one on MX28, thus this
is only a few changes fixing the parts that are specific to MX23.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>