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

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>