pinctrl: armada-37xx: constify pinconf_ops, pinctrl_ops, and pinmux_ops structures

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

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>