Split out the stage 1 pte bits and add the stage 2 bits

In preperation for adding bhyve support to arm64 we need to split the
stage 1 and stage 2 pte fields to allow future changes to create stage 2
page tables.

MFC after: 1 month
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D23669