Tegra SoC Tachometer controller

Required properties:
- compatible: For Tegra186, must contain "nvidia,pwm-tegra186-tachometer".
- reg: physical base address and length of the controller's registers
- #pwm-cells: should be 2. See pwm.txt in this directory for a description of
  the cells format.
- clocks: Must contain one entry, for the module clock.
  See ../clocks/clock-bindings.txt for details.
  Must have the clock name as "tach".
- resets: Must contain an entry for each entry in reset-names.
  See ../reset/reset.txt for details.
- reset-names: Must include the following entries:
  - tach
- pulse-per-rev: Pulse per revolutions. This is based on the Fan property.
- capture-window-length: Window length for capturing PWM signal.


Optional properties:
- enable-dynamic-clock-gating: Enable dynamic clock gating. Once register read,
	clock is disabled.

Example:

	tachometer@39c0000 {
		compatible = "nvidia,pwm-tegra186-tachometer";
		reg = <0x0 0x039c0000 0x0 0x10>;
		clocks = <&tegra_car TEGRA186_CLK_TACH>;
		clock-names = "tach";
		resets = <&tegra_car TEGRA186_RESET_TACH>;
		reset-names = "tach";
		pulse-per-rev = <2>;
		capture-window-length = <2>;
		#pwm-cells = <2>;
	};