Rasperry Pi clock support

Eric Anholt
2015-08-13T23:06:01Z

I'm also avoiding requests to set rate/state when they're a no-op, as
apparently no-op changes to pixel clock break the firmware-configured
display setup. It *shouldn't* change anything, but I've confirmed
that setting the CPU clock works fine, and after a couple of weeks of
prodding pixel clock now, I'm at a loss for what's going on with it.