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