arch: Remove dummy mmiowb() definitions from arch code

Now that no driver code is using mmiowb() directly, remove the dummy
definitions remaining in architectures that don't make use of
asm-generic/io.h, as well as the definition in asm-generic/io.h itself.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

include/asm-generic/io.h

@@ -22,10 +22,6 @@
 #include <asm/mmiowb.h>
 #include <asm-generic/pci_iomap.h>
 
-#ifndef mmiowb
-#define mmiowb() do {} while (0)
-#endif
-
 #ifndef __io_br
 #define __io_br()      barrier()
 #endif