[AVR32] Define mmiowb()

Add empty definition of mmiowb() since some drivers need it. Uncached
writes are strongly ordered on AVR32. They may be delayed if the
dcache is busy doing a writeback, but AFAICT that's not what this
macro is supposed to deal with, at least on UP systems.

We might have to revisit this definition when a SMP-capable AVR32 CPU
comes along, depending on how the busses and cache coherency stuff
end up being implemented.

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