pinctrl: palmas: Move fixed assignments to 'pinctrl_desc' definition

Assign 'struct pinctrl_desc' .pins and .npins members in definition to
make clear that number of pins is fixed and have less code in the probe.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/20250611-pinctrl-const-desc-v2-13-b11c1d650384@linaro.org


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