pinctrl: Make of_device_id array const

Make of_device_id array const, because all OF functions handle it as
const.

Signed-off-by: Kiran Padwal <kiran.padwal21@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>