[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>

include/asm-avr32/io.h

@@ -255,6 +255,8 @@ static inline void memset_io(volatile void __iomem *addr, unsigned char val,
 	memset((void __force *)addr, val, count);
 }
 
+#define mmiowb()
+
 #define IO_SPACE_LIMIT	0xffffffff
 
 extern void __iomem *__ioremap(unsigned long offset, size_t size,