linux/io.h: add generic ioremap()/iounmap() defines

linux/io.h: add generic ioremap()/iounmap() defines

For most of architectures in U-Boot, virtual address is straight
mapped to physical address.  So, it makes sense to have generic
defines of ioremap and friends in <linux/io.h>.

All of them are just empty and will disappear at compile time, but
they will be helpful to implement drivers which are counterparts of
Linux ones.

I notice MIPS already has its own implementation, so I added a
Kconfig symbol CONFIG_HAVE_ARCH_IOREMAP which MIPS (and maybe
Sandbox as well) can select.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>