MTK SPI device

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
    - mediatek,mt8173-spi: for mt2701 platforms

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

- interrupts: Should contain spi interrupt

- clock-names: tuple listing input clock names.
	Required elements: "main"

- clocks: phandles to input clocks.

- pad-select: should specify spi pad used, only required for MT8173.
	This value should be 0~3.

Example:

- SoC Specific Portion:
spi: spi@1100a000 {
	compatible = "mediatek,mt8173-spi";
	reg = <0 0x1100a000 0 0x1000>;
	interrupts = <GIC_SPI 110 IRQ_TYPE_LEVEL_LOW>;
	clocks = <&pericfg PERI_SPI0>;
	clock-names = "main";
	pad-select = <1>;
	status = "disabled";
};