Rename the "event_enable_bitmap" field of the GSI structure to be
"ieob_enabled_bitmap".  An upcoming patch will cache the last value
stored for another interrupt mask and this is a more direct naming
convention to follow.

Add a few comments to explain the bitmap fields in the GSI structure.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>