Calling ov5645_s_power() will power-up or power-down this OV5645. And I found that the program gst-launch-1.0 may call ov5645_s_power() internally, but I just don't know how it can do this. I'm wondering whether there is anyway I can use any V4L2 control (maybe something like V4L2_CID_***?) to get ov5645_s_power() to be called, so I can arbitrarily power this OV5645 up/down?
I'm not sure whether this question is a proper one for this forum or not. If not, could anyone advice a better place for this question?
This sadly isn't the right forum, no. The question you're asking is a very specific question about a very specific embedded system that isn't borderline ubiquitous. If there is a forum dedicated to Dragonboard enthusiasts (or even embedded programming in general), that might be a better place to ask.
That said, it's possible that there's some way to get that ov5645_core_ops struct, possibly by some function returning a pointer to it. It might be worth digging around gst-launch's source code to see how it does it.