random(4): Reorder configuration of random source modules

Move fast entropy source registration to the earlier
SI_SUB_RANDOM:SI_ORDER_FOURTH and move random_harvestq_prime after that.
Relocate the registration routines out of the much later randomdev module
and into random_harvestq.

This is necessary for the fast random sources to actually register before we
perform random_harvestq_prime() early in the kernel boot.

No functional change.

Reviewed by: delphij, markjm
Approved by: secteam(delphij)
Differential Revision: https://reviews.freebsd.org/D21308