tile: 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 tile, in the
same vein as the dummy definitions for the relaxed read accessors.
Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/tile/include/asm/io.h

@@ -241,6 +241,10 @@ static inline void writeq(u64 val, unsigned long addr)
 #define readw_relaxed readw
 #define readl_relaxed readl
 #define readq_relaxed readq
+#define writeb_relaxed writeb
+#define writew_relaxed writew
+#define writel_relaxed writel
+#define writeq_relaxed writeq
 
 #define ioread8 readb
 #define ioread16 readw