iio:adis16400: Add support for the adis16448

The adis16448 is more or less from the same family of devices as supported by
this driver. It features three acceleration channels, three angular velocity
channels, three magnetometer channels, one temperature channels and one
barometric pressure channel.

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