xtensa: Use generic asm/mmu.h for nommu

The nommu portion of mmu.h of the extensa platform is basically the same as the
asm-generic mmu.h. So use it instead.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>