There's no way to bind targeting of specific subsystems to a key, I suspect this is by design as it's been brought up before and no action has been taken.
You can bind a key to "Target Next Subsystem" (and another to target previous) which will allow you to quickly cycle through the subsystems until you get the one you want. I have it bound to a button + hat right, and use it all the time.
The other thing you can do, if you're on PC, is pick up Voice Attack, and create a macro that hits the key you bound for "target next subsystem" several times in quick succession. As different ships have different subsystems, you can't get the macro to land on power plant only, but it can skip the first 6-7 subsystems for you, and then you only need to hit your bind 1-2 times. I used to have a macro like this, but I found it more effort than it was worth. You may have more luck though.