pinctrl: pistachio: Constify static 'pinctrl_desc'

The local static 'struct pinctrl_desc' is not modified, so can be made
const for code safety after moving .pins and .npins assignment to
definition.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/20250611-pinctrl-const-desc-v2-15-b11c1d650384@linaro.org


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