[AVR32] Add PIOE device and reserve SDRAM pins

The PIOE device was left out before because it muxes SDRAM pins (and
is therefore a bit dangerous to mess with) and because no existing
drivers had any use for it.

It is needed for CompactFlash, however, and now that we have a way
to protect the SDRAM pins, it can be safely added.

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