staging:iio:gyro: add adis16251 support to adis16260 driver

These parts are very similar and the adis16260 driver supports
a lot of additional functionality.

Precursor to removal of adis16251 driver.

Note that some supported devices were missing from Kconfig help text
and are also added in this patch.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>