pinctrl: aspeed: Constify static 'pinctrl_desc'

pinctrl: aspeed: Constify static 'pinctrl_desc'

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

Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/20250611-pinctrl-const-desc-v2-5-b11c1d650384@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>