iio: Add a comment to about alphabetical order to Kconfigs and Makefiles

Keeping Makefile and Kconfig entries in alphabetical order usually works better
than just appending new entries at the end, since it reduces the amount of
conflicts. This patch adds a comment to the IIO Kconfig and Makefile files to
document that the entries should be kept in alphabetical order.

Also reorder those  entries which weren't in alphabetical order yet.

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