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

pinctrl: renesas: 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.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-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-v2-14-b11c1d650384@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>