random(4): Reorder configuration of random source modules

[cem]

Summary

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.