NVIDIA Tegra124 SPI Slave controller.

Required properties:
- compatible : should be "nvidia,tegra124-spi-slave".
- reg: Should contain SPI registers location and length.
- interrupts: Should contain SPI interrupts.
- nvidia,dma-request-selector : The Tegra DMA controller's phandle and
  request selector for this SPI controller.

Recommended properties:
- spi-max-frequency: Maximum SPI clocking speed of device in Hz. For slave,
	It must be at least 1.5 times bus rate.

Optional properties:
- nvidia,clock-always-on: Enable clock of spi always.
- nvidia,maximum-dma-buffer-size: Maximum dma buffer size per transfer.
	If this is not available then 16K will be default. The value should
	be unit of byte.
- nvidia,slave-ready-gpio: OUT GPIO from slave to report slave status. This
	GPIO, when asserted, implies slave controller is ready for transaction.
	When this GPIO is de-asserted, the master cannot initiate transaction
	with this slave.
- nvidia,lsbyte-first: controls byte order; if enabled, least-significant-byte
        will be transmitted/received first.

Example:
spi@7000d600 {
	compatible = "nvidia,tegra124-spi-slave";
	reg = <0x7000d600 0x200>;
	interrupts = <0 82 0x04>;
	nvidia,dma-request-selector = <&apbdma 16>;
	spi-max-frequency = <25000000>;
	nvidia,clock-always-on;
	#address-cells = <1>;
	#size-cells = <0>;
	status = "disabled";
	nvidia,slave-ready-gpio = <&gpio TEGRA_GPIO(I, 3) GPIO_ACTIVE_LOW>; /*PI3*/
        nvidia,lsbyte-first;
};