dt-binding: spi: add NPCM PSPI controller documentation

Nuvoton NPCM Peripheral Serial Peripheral Interface(PSPI) controller driver

Nuvoton NPCM7xx SOC support two PSPI channels.

Required properties:

 - compatible : "nuvoton,npcm750-pspi" for NPCM7XX BMC

 - #address-cells : should be 1. see spi-bus.txt

 - #size-cells : should be 0. see spi-bus.txt

 - specifies physical base address and size of the register.

 - interrupts : contain PSPI interrupt.

 - clocks : phandle of PSPI reference clock.

 - clock-names: Should be "clk_apb5".

 - pinctrl-names : a pinctrl state named "default" must be defined.

 - pinctrl-0 : phandle referencing pin configuration of the device.

 - cs-gpios: Specifies the gpio pins to be used for chipselects.

            See: Documentation/devicetree/bindings/spi/spi-bus.txt

Optional properties:

- clock-frequency : Input clock frequency to the PSPI block in Hz.

		    Default is 25000000 Hz.

Example:

spi0: spi@f0200000 {

	compatible = "nuvoton,npcm750-pspi";

	reg = <0xf0200000 0x1000>;

	pinctrl-names = "default";

	pinctrl-0 = <&pspi1_pins>;

	#address-cells = <1>;

	#size-cells = <0>;

	interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&clk NPCM7XX_CLK_APB5>;

	clock-names = "clk_apb5";

	cs-gpios = <&gpio6 11 GPIO_ACTIVE_LOW>;

};