xtensa: Implement phys virt conversion for PTP_MMU

For PTP_MMU our physical address is not directly mapped
into virtual address space, we need to access physical
memory from those fixed map segments.

Implement phys_to_virt and virt_to_phys hook to reflect
this setting.

Tested-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>