asm-generic: don't provide ioremap for CONFIG_MMU

All MMU-enabled ports have a non-trivial ioremap and should thus provide
the prototype for their implementation instead of providing a generic
one unless a different symbol is not defined.  Note that this only
affects sparc32 nds32 as all others do provide their own version.

Also update the kerneldoc comments in asm-generic/io.h to explain the
situation around the default ioremap* implementations correctly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>