iio: Move generic buffer implementations to sub-directory

For generic IIO trigger implementations we already have a sub-directory,
but the generic buffer implementations currently reside in the IIO
top-level directory. The main reason is that things have historically grown
into this form.

With more generic buffer implementations on its way now is the perfect time
to clean this up and introduce a sub-directory for generic buffer
implementations to avoid too much clutter in the top-level directory.

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