dt-bindings: thermal: generic-adc: make lookup-table optional

Update binding description making lookup-table optional.
Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

Documentation/devicetree/bindings/thermal/thermal-generic-adc.txt

@@ -8,16 +8,22 @@ temperature using voltage-temperature lookup table.
 Required properties:
 ===================
 - compatible:		     Must be "generic-adc-thermal".
+- #thermal-sensor-cells:     Should be 1. See ./thermal.txt for a description
+		             of this property.
+Optional properties:
+===================
 - temperature-lookup-table:  Two dimensional array of Integer; lookup table
 			     to map the relation between ADC value and
 			     temperature. When ADC is read, the value is
 			     looked up on the table to get the equivalent
 			     temperature.
+
 			     The first value of the each row of array is the
 			     temperature in milliCelsius and second value of
 			     the each row of array is the ADC read value.
-- #thermal-sensor-cells:     Should be 1. See ./thermal.txt for a description
-			     of this property.
+
+			     If not specified, driver assumes the ADC channel
+			     gives milliCelsius directly.
 
 Example :
 #include <dt-bindings/thermal/thermal.h>