asm-generic: add generic NOMMU versions of some headers

Memory management in generic is highly architecture specific,
but on NOMMU architectures, it is mostly trivial, so just
add a default implementation in asm-generic that applies
to all NOMMU architectures.

The two files cache.h and cacheflush.h can possibly also
be used by architectures that have an MMU but never require
flushing the cache or have cache lines larger than 32 bytes.

Signed-off-by: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>