ae: auto-exposure support

This adds simple auto-exposure/gain/white balance support. It was tested on PinePhone, but I believe code is simple enough to work at Librem 5 as well. Going from total darkness to bright sunlight and vice-versa takes ~12 seconds, and could be improved, but I believe this is already better than setting things manually. It is basically a pre-requisite for auto-focus support; I already have an implementation but I'm less confident about it so I'm submitting this first.