sh: add writesb(), readsb(), writesw() and readsw() to io.h

This patch adds inline versions of writesb(), readsb(), writesw() and
readsw() to include/asm-sh/io.h. Stolen from include/asm-avr32/io.h.

These functions are needed to compile certain device drivers such as
ax88796.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>