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";
};
