xtensa: nommu: don't build most of the cache flushing code

Most cache flushing code is only relevant for MMU. Don't build it for
nommu configuration.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

diff --cc arch/xtensa/include/asm/cacheflush.h
Simple merge

diff --cc arch/xtensa/include/asm/page.h
Simple merge

diff --cc arch/xtensa/mm/Makefile
Simple merge