dt-bindings: MIPS: Document xilfpga bindings and boot style

Xilfpga boots only with device-tree. Document the required properties
and the unique boot style
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: robh+dt@kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11361/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/mips/img/xilfpga.txt

+Imagination University Program MIPSfpga
+=======================================
+
+Under the Imagination University Program, a microAptiv UP core has been
+released for academic usage.
+
+As we are dealing with a MIPS core instantiated on an FPGA, specifications
+are fluid and can be varied in RTL.
+
+This binding document is provided as baseline guidance for the example
+project provided by IMG.
+
+The example project runs on the Nexys4DDR board by Digilent powered by
+the ARTIX-7 FPGA by Xilinx.
+
+Relevant details about the example project and the Nexys4DDR board:
+
+- microAptiv UP core m14Kc
+- 50MHz clock speed
+- 128Mbyte DDR RAM	at 0x0000_0000
+- 8Kbyte RAM		at 0x1000_0000
+- axi_intc		at 0x1020_0000
+- axi_uart16550		at 0x1040_0000
+- axi_gpio		at 0x1060_0000
+- axi_i2c		at 0x10A0_0000
+- custom_gpio		at 0x10C0_0000
+- axi_ethernetlite	at 0x10E0_0000
+- 8Kbyte BootRAM	at 0x1FC0_0000
+
+Required properties:
+--------------------
+ - compatible: Must include "digilent,nexys4ddr","img,xilfpga".
+
+CPU nodes:
+----------
+A "cpus" node is required.  Required properties:
+ - #address-cells: Must be 1.
+ - #size-cells: Must be 0.
+A CPU sub-node is also required for at least CPU 0. Required properties:
+ - device_type: Must be "cpu".
+ - compatible: Must be "mips,m14Kc".
+ - reg: Must be <0>.
+ - clocks: phandle to ext clock for fixed-clock received by MIPS core.
+
+Example:
+
+	compatible = "img,xilfpga","digilent,nexys4ddr";
+	cpus {
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		cpu0: cpu@0 {
+			device_type = "cpu";
+			compatible = "mips,m14Kc";
+			reg = <0>;
+			clocks	= <&ext>;
+		};
+	};
+
+	ext: ext {
+		compatible = "fixed-clock";
+		#clock-cells = <0>;
+		clock-frequency = <50000000>;
+	};
+
+Boot protocol:
+--------------
+
+The BootRAM is a writeable "RAM" in FPGA at 0x1FC0_0000.
+This is for easy reprogrammibility via JTAG.
+
+The BootRAM initializes the cache and the axi_uart peripheral.
+
+DDR initialization is already handled by a HW IP block.
+
+When the example project bitstream is loaded, the cpu_reset button
+needs to be pressed.
+
+The bootram initializes the cache and axi_uart.
+Then outputs MIPSFPGA\n\r on the serial port on the Nexys4DDR board.
+
+At this point, the board is ready to load the Linux kernel
+vmlinux file via JTAG.