xtensa: fix noMMU build on cores with MMU

Commit bf15f86b

 ("xtensa: initialize MMU before jumping to reset
vector") calls MMU management functions even when CONFIG_MMU is not
selected. That breaks noMMU build on cores with MMU.

Don't manage MMU when CONFIG_MMU is not selected.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>