Documentation/gpu: Fix Panthor documentation build warnings

Fix Panthor documentation build errors uncovered by the makedocs target
when building with extra warnings enabled.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Fixes: 6a797bdf ("drm/panthor: add sysfs knob for enabling job profiling")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241009214346.2308917-1-adrian.larumbe@collabora.com