Add Allwinner A83T thermal sensor controller support.

The A83T thermal sensor controller has three sensors. Sensor 0 corresponds
to CPU cluster 0, sensor 1 to CPU cluster 1, and sensor 2 to the GPU. This
driver exports the temperature sensor readings via sysctl.

Calibration data is obtained from SRAM found in the Secure ID module.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D6378