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

This patch updates document devicetree bindings
to support multiple devices.

Signed-off-by: Leilk Liu <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-mt65xx.txt

···
   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";
 };