dt-bindings: mfd: Add Spreadtrum SC27xx PMIC documentation

This patch adds the binding documentation for Spreadtrum SC27xx series
PMIC device.
Signed-off-by: NBaolin Wang <baolin.wang@spreadtrum.com>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/sprd,sc27xx-pmic.txt

+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>;
+};