Uses of struct of_device_id are most commonly const.

Suggest using it as such.
Signed-off-by: NJoe Perches <joe@perches.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>