arm64 pmap: introduce PHYS_TO_PTE macro

Introduce macro for PHYS_TO_PTE, 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 PHYS_TO_PTE is a NOP. Replace all instances where we go from
PA to PTE with new PHYS_TO_PTE macro.

Reviewed by: markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D39828