microblaze: Define read/write{b,w,l}_relaxed MMIO

More and more ARM specific drivers is using MMIO
readX/writeX_relaxed IO functions and Microblaze can
shared some drivers with ARM too.

This patch adds relaxed IO accessor macros
to prevent compilation failures.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/include/asm/io.h

@@ -342,4 +342,12 @@ static inline void outsl(unsigned long addr, const void *buffer, int count)
 #define iowrite32_rep(p, src, count) \
 	outsl((unsigned long) (p), (src), (count))
 
+#define readb_relaxed	readb
+#define readw_relaxed	readw
+#define readl_relaxed	readl
+
+#define writeb_relaxed	writeb
+#define writew_relaxed	writew
+#define writel_relaxed	writel
+
 #endif /* _ASM_MICROBLAZE_IO_H */