drm/panthor: add sysfs knob for enabling job profiling

This commit introduces a DRM device sysfs attribute that lets UM control
the job accounting status in the device. The knob variable had been brought
in as part of a previous commit, but now we're able to fix it manually.

As sysfs files are part of a driver's uAPI, describe its legitimate input
values and output format in a documentation file.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240923230912.2207320-6-adrian.larumbe@collabora.com