drm: Add gpu.tmpl docbook to MAINTAINERS entry

Patches get misrouted and lost otherwise. And use * to future-proof
for sphinx (or whatever the documentation nirvana toolchain will be).

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462225780-3663-1-git-send-email-daniel.vetter@ffwll.ch