thermal: of-thermal: add support for reading coefficients property

In order to avoid having each driver adding their own
specific DT property to specify slope and offset,
this patch adds a basic coefficient reading from
DT thermal zone node. Right now, as the thermal
framework does not support multiple sensors,
the current coefficients apply only to the only
sensor in the thermal zone.

The supported equation is a simple linear model:
	slope * <sensor reading> + offset.

slope and offset are read from the coefficients
DT property. In the same way as it is described in
the DT thermal binding.

So, as of today, the thermal framework will support
only cases like:
                /* hotspot = 1 * adc + 6000 */
		coefficients =          <1      6000>;

Signed-off-by: Eduardo Valentin <edubezval@gmail.com>