dt-bindings: mfd: Add Spreadtrum SC27xx PMIC documentation

Spreadtrum SC27xx Power Management Integrated Circuit (PMIC)

The Spreadtrum SC27xx series PMICs contain SC2720, SC2721, SC2723, SC2730

and SC2731. The Spreadtrum PMIC belonging to SC27xx series integrates all

mobile handset power management, audio codec, battery management and user

interface support function in a single chip. It has 6 major functional

blocks:

- DCDCs to support CPU, memory.

- LDOs to support both internal and external requirement.

- Battery management system, such as charger, fuel gauge.

- Audio codec.

- User interface function, such as indicator, flash LED and so on.

- IC level interface, such as power on/off control, RTC and typec and so on.

Required properties:

- compatible: Should be one of the following:

	"sprd,sc2720"

	"sprd,sc2721"

	"sprd,sc2723"

	"sprd,sc2730"

	"sprd,sc2731"

- reg: The address of the device chip select, should be 0.

- spi-max-frequency: Typically set to 26000000.

- interrupts: The interrupt line the device is connected to.

- interrupt-controller: Marks the device node as an interrupt controller.

- #interrupt-cells: The number of cells to describe an PMIC IRQ, must be 2.

- #address-cells: Child device offset number of cells, must be 1.

- #size-cells: Child device size number of cells, must be 0.

Example:

pmic@0 {

	compatible = "sprd,sc2731";

	reg = <0>;

	spi-max-frequency = <26000000>;

	interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;

	interrupt-controller;

	#interrupt-cells = <2>;

	#address-cells = <1>;

	#size-cells = <0>;

};