asm-generic: add MMU variants of io.h functions

Some of the implementations, in particular the ioremap variants, in
asm-generic/io.h are for systems without an MMU.  In order to be able to
use the generic header file for systems with an MMU, this patch wraps
these implementations in checks for CONFIG_MMU.

Tested on OpenRISC.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Cc: liqin.chen@sunplusct.com
Cc: gxt@mprc.pku.edu.cn
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>