arch/xtensa: Rename xtensa_mmu.c to ptables.c

The job of this file is almost exclusively page table management.  MMU
hardware interaction is in there but a small part, and they don't
interact except in a few small APIs.  Rename it in advance of doing
work to the hardware side.

Signed-off-by: Andy Ross <andyross@google.com>

Author: andyross