drm/amdgpu: add DMUB outbox event IRQ source define/complete/debug flag

[Why & How]
We use outbox interrupt that allows us to do the AUX via DMUB
Therefore, we need to add some irq source related definition
in the header files;

Signed-off-by: Jude Shih <shenshih@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>