iio: at91: introduce the multiple compatible string for different IPs.

As use the multiple compatible string, we can remove hardware register in dt.

CC: devicetree@vger.kernel.org
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>