Allwinner A31 SPI controller

Required properties:
- compatible: Should be "allwinner,sun6i-a31-spi".
- reg: Should contain register location and length.
- interrupts: Should contain interrupt.
- clocks: phandle to the clocks feeding the SPI controller. Two are
          needed:
  - "ahb": the gated AHB parent clock
  - "mod": the parent module clock
- clock-names: Must contain the clock names described just above
- resets: phandle to the reset controller asserting this device in
          reset

Example:

spi1: spi@01c69000 {
	compatible = "allwinner,sun6i-a31-spi";
	reg = <0x01c69000 0x1000>;
	interrupts = <0 66 4>;
	clocks = <&ahb1_gates 21>, <&spi1_clk>;
	clock-names = "ahb", "mod";
	resets = <&ahb1_rst 21>;
};