pinctrl: Constify pointers to 'pinctrl_desc'

pinctrl: Constify pointers to 'pinctrl_desc'

Pin controller core code only stores the pointer to
'struct pinctrl_desc' and does not modify it anywhere.  The pointer can
be changed to pointer to const which makes the code safer, explicit and
later allows constifying 'pinctrl_desc' allocations in individual
drivers.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/20250611-pinctrl-const-desc-v2-4-b11c1d650384@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>