GPIO: gpio-pxa: simplify pxa_gpio_to_irq() and pxa_irq_to_chip()

Simplify the code in gpio-pxa.c and make them based on irq_base.
When not probed from devicetree, initialize irq_base from
PXA_GPIO_TO_IRQ() or MMP_GPIO_TO_IRQ(), respectively, so the non-DT case
still works.

Only tested on PXA3xx.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>