Device tree bindings for fan tach output connected to IRQ capable GPIO.

Required properties:
- compatible	: "nvidia,gpio-tachometer"
- win-len	: Duration in multiples of 100ms, for which the IRQ will be enabled
		  while measuring speed. Maximum allowed value is 5 seconds to avoid
		  possible IRQ overload at high speed.
- pulse-per-rev	: Number of signals generated on the tach output pin by the motor
		  for one full rotation.
- gpio		: GPIO pin on which tach output of the motor is connected.

Optional properties:
- schedule-delay: Time in milliseconds for the background thread to monitor the speed.
		  If the measured speed is 0, system shutdown will be initiated.
		  Time duration should be higher than win-len time.

Example:
        gpio_tachometer {
                compatible = "nvidia,gpio-tachometer";
                win-len = <2>;			/*For every measurement, take samples for 200ms*/
                pulse-per-rev = <2>;		/*Motor generates 2 pulses for every full rotation*/
                schedule-delay = <10000>;	/*Check motor speed every 10seconds and shutdown if motor is stopped*/
                gpio = <&tegra_main_gpio TEGRA_MAIN_GPIO(Y, 1) 0>;
                status = "okay";
        };