iio: Add macro to populate struct iio_map array

The GPIO core provides a handy GPIO_LOOKUP() macro to populate a struct
gpiod_lookup array without having to spell out attribute names (but
still avoid breakage when attributes within the struct are rearranged
or added).

The axp288_adc.c driver uses a similar macro to populate a struct
iio_map array.  Make it available to others.

Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>