IIO: Add basic MXS LRADC driver

This driver is very basic. It supports userland trigger, buffer and
raw access to channels. The support for delay channels is missing
altogether.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Juergen Beisert <jbe@pengutronix.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Wolfgang Denk <wd@denx.de>