iio: add Kconfig selects needed for triggered buffer compiles

Select IIO_BUFFER and IIO_TRIGGERED_BUFFER to compile.
Remove IIO_TRIGGER if present since IIO_BUFFER selects it.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>