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.