staging:iio:max1363 - move to channel_spec registration.

V3: move to single chan registration macro. Also introduce some
local macros to greatly reduce code length when setting up the
chan_spec arrays for all the different devices.

V2: update read_raw for two value approach.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>