m68k: 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 m68k, in the
same vein as the dummy definitions for the relaxed read accessors.
Additionally, the existing relaxed read accessors are moved into
asm/io.h, so that they can be used by m68k targets with an MMU.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>