iio:imu:adis: Add support for 32bit registers

Some of the newer generation devices from the ADIS16XXX family have 32bit wide
register which spans two 16bit wide registers. This patch adds support for
reading and writing a 32bit wide register.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>