linux/io.h: import generic ioread* / iowrite* accessors from Linux

Some drivers in Linux (ex. drivers/mtd/nand/denali.c) use
ioread*/iowrite* accessors.  Import them to make drivers more
synced.  I copied code from include/asm-generic/io.h of Linux.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>