xtensa: mm/cache: add missing EXPORT_SYMBOLs

commit bc652eb6a0d5cffaea7dc8e8ad488aab2a1bf1ed upstream.

Functions clear_user_highpage, copy_user_highpage, flush_dcache_page,
local_flush_cache_range and local_flush_cache_page may be used from
modules. Export them.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>