MIPS: define write{b,w,l,q}_relaxed

MIPS does define read{b,w,l,q}_relaxed but does not define their write
counterparts: write{b,w,l,q}_relaxed. This patch adds the missing
definitions for the write*_relaxed I/O accessors.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Patchwork: https://patchwork.linux-mips.org/patch/5352/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>