iio: mxs-lradc: add datasheet name for every usable channel

In order to provide a channel name to in kernel consumers add the
datasheet names for every usable AD channel. Since the channel names
differ between i.MX23 and i.MX28, we need to separate the channel
specs.

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