thermal: armada: Add infrastructure to support generic formulas

In order to support other similar SoC, with different sensor
coefficients, this commit adds the coeficients to the per-variant
structure, instead of having the formula hardcoded.

Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>