pinctrl: spacemit: destroy mutex at driver detach

The mutex initialized in probe() is never cleaned up. Use
devm_mutex_init() to destroy it automatically.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Yixun Lan <dlan@gentoo.org>
Link: https://lore.kernel.org/20250305102710.52762-1-brgl@bgdev.pl


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