ledscape_wait CPU usage 99% about ledscape

Ok, so the patch is for a very old version of LEDscape. I tried to shoehorn it in, but it's failing. Still I think using the PRU interrupt might be a better approach that a spinning for loop.

[Yona-Appletree]

I have included an initial version of this interrupt-based waiting. It's not perfect because synchronizing between the PRUs get's a bit tricky and I don't, under any circumstances, want to get into a state where the interrupt has been fired but the userspace code is still waiting. As such, this fix is imperfect, but does decrease CPU usage somewhat (~85% IIRC). I'm going to close this issue, but I'm open to pull requests to improve the behavior.