pinctrl: constify of_device_id array

of_device_id is always used as const.
(See driver.of_match_table and open firmware functions)

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Acked-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>