Binding for MTK SPI controller (MT7621 MIPS)

Required properties:
- compatible: Should be one of the following:
  - "ralink,mt7621-spi": for mt7621/mt7628/mt7688 platforms
- #address-cells: should be 1.
- #size-cells: should be 0.
- reg: Address and length of the register set for the device
- resets: phandle to the reset controller asserting this device in
          reset
  See ../reset/reset.txt for details.

Optional properties:
- cs-gpios: see spi-bus.txt.

Example:

- SoC Specific Portion:
spi0: spi@b00 {
	compatible = "ralink,mt7621-spi";
	reg = <0xb00 0x100>;
	#address-cells = <1>;
	#size-cells = <0>;
	resets = <&rstctrl 18>;
	reset-names = "spi";
};