iio:dummy: Add extra paragraphs on Kconfig

This patch fixes the checkpatch.pl warning:

drivers/iio/dummy/Kconfig:21: WARNING: please write a paragraph that
describes the config symbol fully
drivers/iio/dummy/Kconfig:27: WARNING: please write a paragraph that
describes the config symbol fully

This patch expands the explanation about IIO_DUMMY_EVGEN by using the
code documentation found in iio/dummy/iio_dummy_evgen.c. In the same
way, the information related to IIO_SIMPLE_DUMMY_BUFFER was extracted
from file iio_simple_dummy_buffer.c.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>