pinctrl: ssbi-mpp: constify copied structure

The pm8xxx_pinctrl_desc structure is only copied into another structure,
so make it const.

The opportunity for this change was found using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

Link: https://lore.kernel.org/r/1577864614-5543-9-git-send-email-Julia.Lawall@inria.fr


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>