pinctrl: Constify static 'pinctrl_desc'

The local static 'struct pinctrl_desc' is not modified, so can be made
const for code safety.

Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Hal Feng <hal.feng@starfivetech.com>
Acked-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Link: https://lore.kernel.org/20250611-pinctrl-const-desc-v2-16-b11c1d650384@linaro.org


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