This was a failure of "git add" on my part -- we already referenced
the doc from drivers.rst.

Signed-off-by: Eric Anholt
Cc: Daniel Vetter
Link: https://patchwork.freedesktop.org/patch/msgid/20180703170515.6298-3-eric@anholt.net
Reviewed-by: Alex Deucher
Acked-by: Daniel Vetter