NVIDIA Tegra20/Tegra30 high speed (DMA based) UART controller driver.

Required properties:
- compatible : should be "nvidia,tegra30-hsuart", "nvidia,tegra20-hsuart".
- reg: Should contain UART controller registers location and length.
- interrupts: Should contain UART controller interrupts.
- nvidia,dma-request-selector : The Tegra DMA controller's phandle and
  request selector for this UART controller.

Optional properties:
- nvidia,enable-modem-interrupt: Enable modem interrupts. Should be enable
		only if all 8 lines of UART controller are pinmuxed.

Example:

serial@70006000 {
	compatible = "nvidia,tegra30-hsuart", "nvidia,tegra20-hsuart";
	reg = <0x70006000 0x40>;
	reg-shift = <2>;
	interrupts = <0 36 0x04>;
	nvidia,dma-request-selector = <&apbdma 8>;
	nvidia,enable-modem-interrupt;
	status = "disabled";
};