arm64 pmap: Simplify logic around pv_chunk sizes.

• Define PC_FREEL and _NPCM in terms of _NPCPV rather than via magic numbers.

• Remove assertions about _NPC* values from pmap.c. This is less relevant now that PC_FREEL and _NPCM are derived from _NPCPV.

• Add a helper inline function pc_is_full() which uses a loop to check if pc_map is all zeroes. Use this to replace three places that check for a full mask assuming there are only 3 entries in pc_map.

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D36217