Binding for MTK SPI controller

Required properties:
- compatible: should be one of the following.
    - mediatek,mt8173-spi: for mt8173 platforms
    - mediatek,mt8135-spi: for mt8135 platforms
    - mediatek,mt6589-spi: for mt6589 platforms

- #address-cells: should be 1.

- #size-cells: should be 0.

- reg: Address and length of the register set for the device

- interrupts: Should contain spi interrupt

- clocks: phandles to input clocks.
  The first should be <&topckgen CLK_TOP_SPI_SEL>.
  The second should be one of the following.
   -  <&clk26m>: specify parent clock 26MHZ.
   -  <&topckgen CLK_TOP_SYSPLL3_D2>: specify parent clock 109MHZ.
				      It's the default one.
   -  <&topckgen CLK_TOP_SYSPLL4_D2>: specify parent clock 78MHZ.
   -  <&topckgen CLK_TOP_UNIVPLL2_D4>: specify parent clock 104MHZ.
   -  <&topckgen CLK_TOP_UNIVPLL1_D8>: specify parent clock 78MHZ.

- clock-names: shall be "spi-clk" for the controller clock, and
  "parent-clk" for the parent clock.

Optional properties:
- mediatek,pad-select: specify which pins group(ck/mi/mo/cs) spi
  controller used, this value should be 0~3, only required for MT8173.
    0: specify GPIO69,70,71,72 for spi pins.
    1: specify GPIO102,103,104,105 for spi pins.
    2: specify GPIO128,129,130,131 for spi pins.
    3: specify GPIO5,6,7,8 for spi pins.

Example:

- SoC Specific Portion:
spi: spi@1100a000 {
	compatible = "mediatek,mt8173-spi";
	#address-cells = <1>;
	#size-cells = <0>;
	reg = <0 0x1100a000 0 0x1000>;
	interrupts = <GIC_SPI 110 IRQ_TYPE_LEVEL_LOW>;
	clocks = <&topckgen CLK_TOP_SPI_SEL>, <&topckgen CLK_TOP_SYSPLL3_D2>;
	clock-names = "spi-clk", "parent-clk";
	mediatek,pad-select = <0>;
	status = "disabled";
};