spi: pic32: Add bindings for PIC32 SPI peripheral

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";

};