spi: pic32: Add bindings for PIC32 SPI peripheral

Document the devicetree bindings for the SPI peripheral found
on Microchip PIC32 class devices.
Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/microchip,spi-pic32.txt

+Microchip PIC32 SPI Master controller
+
+Required properties:
+- compatible: Should be "microchip,pic32mzda-spi".
+- reg: Address and length of register space for the device.
+- interrupts: Should contain all three spi interrupts in sequence
+              of <fault-irq>, <receive-irq>, <transmit-irq>.
+- interrupt-names: Should be "fault", "rx", "tx" in order.
+- clocks: Phandle of the clock generating SPI clock on the bus.
+- clock-names: Should be "mck0".
+- cs-gpios: Specifies the gpio pins to be used for chipselects.
+            See: Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Optional properties:
+- dmas: Two or more DMA channel specifiers following the convention outlined
+        in Documentation/devicetree/bindings/dma/dma.txt
+- dma-names: Names for the dma channels. There must be at least one channel
+             named "spi-tx" for transmit and named "spi-rx" for receive.
+
+Example:
+
+spi1: spi@1f821000 {
+        compatible = "microchip,pic32mzda-spi";
+        reg = <0x1f821000 0x200>;
+        interrupts = <109 IRQ_TYPE_LEVEL_HIGH>,
+                     <110 IRQ_TYPE_LEVEL_HIGH>,
+                     <111 IRQ_TYPE_LEVEL_HIGH>;
+        interrupt-names = "fault", "rx", "tx";
+        clocks = <&PBCLK2>;
+        clock-names = "mck0";
+        cs-gpios = <&gpio3 4 GPIO_ACTIVE_LOW>;
+        dmas = <&dma 134>, <&dma 135>;
+        dma-names = "spi-rx", "spi-tx";
+};