arm64 pmap: introduce PTE_TO_PHYS macro

Introduce macro for PTE_TO_PHYS, setting the groundwork for future
support of various Arm VMSA extensions.

For extensions such as 52-bit VA/PA (FEAT_LPA2), the representation of
an address between a PTE and PA are not equivalent. This macro will
allow converting between the different representations.

Currently going from PTE to PA is achieved by masking off the upper and
lower attributes. Retain this behaviour but replace all instances with
the new macro instead.

Reviewed by: alc, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D39827