arm64, qoriq_therm: configure the number of sites base don SoC

Configure the number of sites (sensors) based on SoC.
This avoids timeouts reading non-existent sensors.

The changes are based on mmel's initial work at:
https://github.com/strejda/freebsd/commit/914e3f0098b090cb5c1492b0d24992012c5c553b

MFC after: 1 week
Sponsored by: Traverse Technologies (providing Ten64 HW for testing)
Reviewed by: mmel
Differential Revision: https://reviews.freebsd.org/D35759