staging:iio:adis16400: Add helper macros for channel declaration

Most of the channels declared in the adis16400 driver look quite similar. This
patch adds a bunch of helper macros to initialize the channel spec for this
driver. This allows us to drastically reduce the number of lines of code needed
for the channel spec declaration.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>