Ability to set HDMI output refresh rate

[pizzarollsroyce]

I don't believe that Circle exposes a way to set the HDMI video output refresh rate to 50hz, which I'd like for a project I'm working on.

I tried looking for a mailbox register I could poke to do this, but I'm not able to find anything.

I'll note that if I boot into Raspbian, I can set it to 1280x720@50hz, so the hardware appears to support this.