xtensa: use asm-generic/mmu_context.h for no-op implementations

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>