arm, acpi: increase size of memory region arrays

Bump up MAX_HWCNT and MAX_EXCNT to 32 when ACPI is enabled. These are
the sizes of the hwregions and exregions arrays respectively. ACPI
firmware typically has more memory regions and the current value of
16 is not sufficient for some platforms.

This commit fixes a failure seen with AMI firmware on Cavium's Sabre
ThunderX2 reference platform. This platform needs 21 physical memory
regions and 18 excluded regions to boot correctly with the current
firmware release.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D19073