iio: dummy: iio_dummy_evgen: Demote file header and supply description for 'irq_sim_domain'

File headers are not good candidates for kerneldoc.

Fixes the following W=1 kernel build warning(s):

 drivers/iio/dummy/iio_dummy_evgen.c:30: warning: Cannot understand  * @regs: irq regs we are faking
 on line 30 - I thought it was a doc line
 drivers/iio/dummy/iio_dummy_evgen.c:42: warning: Function parameter or member 'irq_sim_domain' not described in 'iio_dummy_eventgen'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200716135928.1456727-16-lee.jones@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>