xtensa: move MPU constants from .data to .ref.rodata

MPU attribute mapping table is R/O, move it from .data to __REFCONST
(as the rest of the _startup code where initialize_cacheattr is used is
in the __REF section). This allows executing initialize_cacheattr before
the data section of the XIP kernel is relocated to its place.

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