Implement PIO based interrupt service handler

[eamars]

Hello Michael, I find some people didn't use PIO based interrupt handler for sam4s correctly. The PIO interrupt example shows that pio_handler() is one of the generic handler, which is called when any of interrupt occurs at PORT_A. People may not realize that pio_handler() is shared across all PORT_A peripherals.

Furthermore, for those who didn't realize how "Dynamic Vector Table" works, would like to add an additional interrupt source at PORT_A, will possibly go wrong by replacing current interrupt handler with the new one. As a result, multiple interrupt sources would trigger the same interrupt whereas they aren't suppose to do so.

To make it clear, I introduced pio_irq_init(), which would register PIO based interrupt handler, is called when corresponding PIO interrupt is generated. Internally, pio_irq_common_handler will find out which PIO generates interrupt based on the information in PIO_IMR and PIO_ISR, according to datasheet.

However I don't have wacky racer board to test with. I have also included an example showing how to use PIO interrupt correctly.