Xilinx Slave Serial SPI FPGA Manager

Xilinx Spartan-6 FPGAs support a method of loading the bitstream over
what is referred to as "slave serial" interface.
The slave serial link is not technically SPI, and might require extra
circuits in order to play nicely with other SPI slaves on the same bus.

See https://www.xilinx.com/support/documentation/user_guides/ug380.pdf

Required properties:
- compatible: should contain "xlnx,fpga-slave-serial"
- reg: spi chip select of the FPGA
- prog_b-gpios: config pin (referred to as PROGRAM_B in the manual)
- done-gpios: config status pin (referred to as DONE in the manual)

Example for full FPGA configuration:

	fpga-region0 {
		compatible = "fpga-region";
		fpga-mgr = <&fpga_mgr_spi>;
		#address-cells = <0x1>;
		#size-cells = <0x1>;
	};

	spi1: spi@10680 {
		compatible = "marvell,armada-xp-spi", "marvell,orion-spi";
		pinctrl-0 = <&spi0_pins>;
		pinctrl-names = "default";
		#address-cells = <1>;
		#size-cells = <0>;
		cell-index = <1>;
		interrupts = <92>;
		clocks = <&coreclk 0>;

		fpga_mgr_spi: fpga-mgr@0 {
			compatible = "xlnx,fpga-slave-serial";
			spi-max-frequency = <60000000>;
			spi-cpha;
			reg = <0>;
			done-gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
			prog_b-gpios = <&gpio0 29 GPIO_ACTIVE_LOW>;
		};
	};