gpio: pxa: use devres for the clock struct

The clock is never released after probe(). Use devres to not leak
resources.

Reported-by: Hulk Robot <hulkci@huawei.com>
Reported-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>