[AVR32] Make I/O access macros work with external devices

Fix the I/O access macros so that they work with externally connected
devices accessed in little-endian mode over any bus width:

* Use a set of macros to define I/O port- and memory operations
  borrowed from MIPS.
* Allow subarchitecture to specify address- and data-mangling
* Implement at32ap-specific port mangling (with build-time
  configurable bus width. Only one bus width at a time supported
  for now.)
* Rewrite iowriteN and friends to use write[bwl] and friends
  (not the __raw counterparts.)

This has been tested using pata_pcmcia to access a CompactFlash card
connected to the EBI (16-bit bus width.)

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