frv: io: implement dummy relaxed accessor macros for writes

write{b,w,l}_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 frv, in the same
vein as the dummy definitions for the relaxed read accessors.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/frv/include/asm/io.h

@@ -243,6 +243,9 @@ static inline void writel(uint32_t datum, volatile void __iomem *addr)
 		__flush_PCI_writes();
 }
 
+#define writeb_relaxed writeb
+#define writew_relaxed writew
+#define writel_relaxed writel
 
 /* Values for nocacheflag and cmode */
 #define IOMAP_FULL_CACHING		0