Even VideoCore 4 with the Mesa vc4 driver supports Wayland — since it's Mesa, of course it implements GBM and whatnot. I've used Weston on an RPi3 with Arch Linux ARM.
For VideoCore 5/6, IIUC the only driver is Mesa v3d, even better situation, of course everything should work, and it will always work since no proprietary crap driver would ever get into people's hands.
