spi: mediatek: Update document devicetree bindings to support multiple devices

  muxes clock, and "spi-clk" for the clock gate.

Optional properties:

-cs-gpios: see spi-bus.txt, only required for MT8173.

- mediatek,pad-select: specify which pins group(ck/mi/mo/cs) spi

  controller used, this value should be 0~3, only required for MT8173.

  controller used. This is a array, the element 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.

		 <&topckgen CLK_TOP_SPI_SEL>,

		 <&pericfg CLK_PERI_SPI0>;

	clock-names = "parent-clk", "sel-clk", "spi-clk";

	mediatek,pad-select = <0>;

	cs-gpios = <&pio 105 GPIO_ACTIVE_LOW>, <&pio 72 GPIO_ACTIVE_LOW>;

	mediatek,pad-select = <1>, <0>;

	status = "disabled";

};