These structures are only stored in fields of a pinctrl_desc
structure (confops, pctlops, and pmxops) that are const. Make the
structures 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>