Introduce armada_thermal driver for Armada family platforms

• Currently supports only Armada38X family but other Marvell SoC's can be added if needed.
• Provides temperature is C deg.
• To print the temperature one can use: sysctl dev.armada_thermal.0.temperature

Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D9217