spi: spi-davinci: Update binding for 66AK2Gx pwr dm property

	based on a specific SoC configuration.

- interrupts: interrupt number mapped to CPU.

- clocks: spi clk phandle

          For 66AK2G this property should be set per binding,

          Documentation/devicetree/bindings/clock/ti,sci-clk.txt

SoC-specific Required Properties:

The following are mandatory properties for Keystone 2 66AK2G SoCs only:

- power-domains:	Should contain a phandle to a PM domain provider node

			and an args specifier containing the SPI device id

			value. This property is as per the binding,

Optional:

- cs-gpios: gpio chip selects