sparc: 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 sparc, in the
same vein as the dummy definitions for the relaxed read accessors. The
existing relaxed read{b,w,l} accessors are moved into asm/io.h, since
they are identical between 32-bit and 64-bit machines.

Acked-by: "David S. Miller" <davem@davemloft.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>