thermal: support slope and offset coefficients

It is common to have a linear extrapolation from
the current sensor readings and the actual temperature
value. This is specially the case when the sensor
is in use to extrapolate hotspots.

This patch adds slope and offset constants for
single sensor linear extrapolation equation. Because
the same sensor can be use in different locations,
from board to board, these constants are added
as part of thermal_zone_params.

The constants are available through sysfs.

It is up to the device driver to determine
the usage of these values.

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

Documentation/thermal/sysfs-api.txt

@@ -190,6 +190,8 @@ Thermal zone device sys I/F, created once it's registered:
     |---k_i:                    PID's integral term in the power allocator gov
     |---k_d:                    PID's derivative term in the power allocator
     |---integral_cutoff:        Offset above which errors are accumulated
+    |---slope:                  Slope constant applied as linear extrapolation
+    |---offset:                 Offset constant applied as linear extrapolation
 
 Thermal cooling device sys I/F, created once it's registered:
 /sys/class/thermal/cooling_device[0-*]:
@@ -359,6 +361,20 @@ integral_cutoff
 	Documentation/thermal/power_allocator.txt
 	RW, Optional
 
+slope
+	The slope constant used in a linear extrapolation model
+	to determine a hotspot temperature based off the sensor's
+	raw readings. It is up to the device driver to determine
+	the usage of these values.
+	RW, Optional
+
+offset
+	The offset constant used in a linear extrapolation model
+	to determine a hotspot temperature based off the sensor's
+	raw readings. It is up to the device driver to determine
+	the usage of these values.
+	RW, Optional
+
 *****************************
 * Cooling device attributes *
 *****************************