spi: Add Spreadtrum ADI controller documentation

This patch adds the binding documentation for Spreadtrum ADI
controller device.
Signed-off-by: Baolin Wang <baolin.wang@spreadtrum.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-sprd-adi.txt

+Spreadtrum ADI controller
+
+ADI is the abbreviation of Anolog-Digital interface, which is used to access
+analog chip (such as PMIC) from digital chip. ADI controller follows the SPI
+framework for its hardware implementation is alike to SPI bus and its timing
+is compatile to SPI timing.
+
+ADI controller has 50 channels including 2 software read/write channels and
+48 hardware channels to access analog chip. For 2 software read/write channels,
+users should set ADI registers to access analog chip. For hardware channels,
+we can configure them to allow other hardware components to use it independently,
+which means we can just link one analog chip address to one hardware channel,
+then users can access the mapped analog chip address by this hardware channel
+triggered by hardware components instead of ADI software channels.
+
+Thus we introduce one property named "sprd,hw-channels" to configure hardware
+channels, the first value specifies the hardware channel id which is used to
+transfer data triggered by hardware automatically, and the second value specifies
+the analog chip address where user want to access by hardware components.
+
+Since we have multi-subsystems will use unique ADI to access analog chip, when
+one system is reading/writing data by ADI software channels, that should be under
+one hardware spinlock protection to prevent other systems from reading/writing
+data by ADI software channels at the same time, or two parallel routine of setting
+ADI registers will make ADI controller registers chaos to lead incorrect results.
+Then we need one hardware spinlock to synchronize between the multiple subsystems.
+
+Required properties:
+- compatible: Should be "sprd,sc9860-adi".
+- reg: Offset and length of ADI-SPI controller register space.
+- hwlocks: Reference to a phandle of a hwlock provider node.
+- hwlock-names: Reference to hwlock name strings defined in the same order
+	as the hwlocks, should be "adi".
+- #address-cells: Number of cells required to define a chip select address
+	on the ADI-SPI bus. Should be set to 1.
+- #size-cells: Size of cells required to define a chip select address size
+	on the ADI-SPI bus. Should be set to 0.
+
+Optional properties:
+- sprd,hw-channels: This is an array of channel values up to 49 channels.
+	The first value specifies the hardware channel id which is used to
+	transfer data triggered by hardware automatically, and the second
+	value specifies the analog chip address where user want to access
+	by hardware components.
+
+SPI slave nodes must be children of the SPI controller node and can contain
+properties described in Documentation/devicetree/bindings/spi/spi-bus.txt.
+
+Example:
+	adi_bus: spi@40030000 {
+		compatible = "sprd,sc9860-adi";
+		reg = <0 0x40030000 0 0x10000>;
+		hwlocks = <&hwlock1 0>;
+		hwlock-names = "adi";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		sprd,hw-channels = <30 0x8c20>;
+	};