Split out creating the arm64 L2 dmap entries

When creating the DMAP region we may need to create level 2 page table
entries at the start and end of a block of memory. The code to do this
was almost identical so we can merge into a single function.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit ba3b60200191399c3c128e9acee47b5254514822)