Extend flash driver with v4l support.

The camera flash's LED driver needs flash support added which i skipped back then since it wasn't simple to test. With the right user space API falling into place this can be added (just a couple lines of code). Likely on my table to finish unfinished work.

(known issue but let's track it)

Prompted by @martin.kepplinger nice https://source.puri.sm/Librem5/linux-next/-/merge_requests/453