Bump MAX_HWCNT and MAX_EXCNT.

These limits are hit on the ThunderX. Also make
arm_physmem_exclude_region() panic rather than fail silently if the
limit on excluded regions is reached.

PR: 231064
Reviewed by: andrew
Approved by: re (kib)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17073