Device-Tree bindings for input/keyboard/adc_keypad.c keyboard driver

Required properties:
- compatible: Must be "amlogic, adc_keypad".
- key_name: Specify the name of different key.
- key_num:  Specify the quantity of the keys.
- key_code: Set the function code corresponding to different key.
- key_chan: Set the channel to detect the key, can be set from 0 to 7.
- key_val: The adc sampling value of different key, can be set from 0 to 1023.
- key_tolerance: tolerance of the adc sampling value, recommend maximum value of 40.

Optional properties:
- status: Shall be "ok" or "okay" if enabled or "disabled" if disabled.
          Default is "ok".

Example:
	adc_keypad{
			compatible = "amlogic, adc_keypad";
			status = "okay";
			key_name = "left", "right","up", "down", "enter";
			key_num = <5>;
			key_code = <106 105 103 108 28>;
			key_chan = <2 2 3 3 3>;
			key_val =  <516 253 516 750 0>; //voltage=0/252/478/692/824mV, val=voltage/1800mV*1023
			key_tolerance = <40 40 40 40 40>;
   };
