pinctrl: artpec6: constify pinconf_ops, pinctrl_ops, and pinmux_ops structures

This pinctrl_ops structure is only stored in the const pctlops
field of a pinctrl_desc structure. Make the pinctrl_ops structure
const as well.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Lars Persson <lars.persson@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>