nds32: asm/io.h: add __iormb __iowmb and inline io support

1. This patch add required __iormb and __iowmb to io.h.
   This also fix some misbehavior to periphal drivers.
   This io.h has been fixed with referencing arm/include/asm/io.h.
2. This patch replaced macro writeb and readb into inline function.

Signed-off-by: Macpaul Lin <macpaul@andestech.com>