Start to support multiple stages in the arm64 pmap.

On arm64 the stage 1 and stage 2 pte formats are similar enough we can
reuse the pmap code for both. As they are only similar and not identical
we need to know if we are managing stage 1 or stage 2 tables.

Add an enum to store this information and a check to make sure it is
set to stage 1 when we manage stage 1 pte fields.

Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D23830