xtensa: mmu: MMU re-initialization API

With power managment is enabled, depending on the SoC power state
used when idle, the MMU may lose context and may need to be re-initialized.
When re-initializing the MMU, we must not re-create the page table
because it may overwrite changes done during the execution, but we still
need to set the asid and page table for the current context.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>