avr32: Introduce <mach/chip.h>

This file includes the appropriate chip-specific header with MMIO, IRQ
and GPIO definitions used by the platform code. It may also be used to
provide inline GPIO accessors for drivers that are willing to sacrifice
portability for faster bitbanging.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>