xtensa: fix warning '"CONFIG_OF" is not defined'

The warning only shows up when building MMUv3 configuration with OF
support disabled.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: fix asm constraints unsupported by xcc

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