arm64: fix free queue and reservation configuration for 16KB pages

Correctly configure the free page queues and the reservation size when
the base page size is 16KB. In particular, the reservation size was
less than the L2 Block size, making L2 promotions and mappings all but
impossible.

Reviewed by: markj
Tested by: gallatin
Differential Revision: https://reviews.freebsd.org/D42737