Add PL310 platform initialization for Armada 38x

Introduce machine-dependent part of the arm/pl310 driver for
Armada 38x SoCs. Add prefetch and power savings configuration.

Submitted by: <arnaud.ysmal@stormshield.eu>
Obtained from: Stormshield
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D10220