powerpc: io: implement dummy relaxed accessor macros for writes

write{b,w,l,q}_relaxed are implemented by some architectures in order to
permit memory-mapped I/O accesses with weaker barrier semantics than the
non-relaxed variants.

This patch adds dummy macros for the write accessors to powerpc, in the
same vein as the dummy definitions for the relaxed read accessors.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/powerpc/include/asm/io.h

@@ -617,10 +617,14 @@ static inline void name at					\
 /*
  * We don't do relaxed operations yet, at least not with this semantic
  */
-#define readb_relaxed(addr) readb(addr)
-#define readw_relaxed(addr) readw(addr)
-#define readl_relaxed(addr) readl(addr)
-#define readq_relaxed(addr) readq(addr)
+#define readb_relaxed(addr)	readb(addr)
+#define readw_relaxed(addr)	readw(addr)
+#define readl_relaxed(addr)	readl(addr)
+#define readq_relaxed(addr)	readq(addr)
+#define writeb_relaxed(v, addr)	writeb(v, addr)
+#define writew_relaxed(v, addr)	writew(v, addr)
+#define writel_relaxed(v, addr)	writel(v, addr)
+#define writeq_relaxed(v, addr)	writeq(v, addr)
 
 #ifdef CONFIG_PPC32
 #define mmiowb()