x86: Document irq router device tree bindings

Describe all required properties needed by the irq router device tree.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>

diff --git a/doc/device-tree-bindings/misc/intel,irq-router.txt b/doc/device-tree-bindings/misc/intel,irq-router.txt
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+Intel Interrupt Router Device Binding
+=====================================
+
+The device tree node which describes the operation of the Intel Interrupt Router
+device is as follows:
+
+Required properties :
+- reg : Specifies the interrupt router's PCI configuration space address as
+    defined by the Open Firmware spec.
+- compatible = "intel,irq-router"
+- intel,pirq-config : Specifies the IRQ routing register programming mechanism.
+    Valid values are:
+      "pci": IRQ routing is controlled by PCI configuration registers
+      "ibase": IRQ routing is in the memory-mapped IBASE register block
+- intel,ibase-offset : IBASE register offset in the interrupt router's PCI
+    configuration space, required only if intel,pirq-config = "ibase".
+- intel,pirq-link : Specifies the PIRQ link information with two cells. The
+    first cell is the register offset that controls the first PIRQ link routing.
+    The second cell is the total number of PIRQ links the router supports.
+- intel,pirq-mask : Specifies the IRQ mask reprenting the 16 IRQs in 8259 PIC.
+    Bit N is 1 means IRQ N is available to be routed.
+- intel,pirq-routing : Specifies all PCI devices' IRQ routing information,
+   encoded as 3 cells a group for a device. The first cell is the device's PCI
+   bus number, device number and function number encoding with PCI_BDF() macro.
+   The second cell is the PCI interrupt pin used by this device. The last cell
+   is which PIRQ line the PCI interrupt pin is routed to.
+
+
+Example
+-------
+
+#include <dt-bindings/interrupt-router/intel-irq.h>
+
+       irq-router@1f,0 {
+               reg = <0x0000f800 0 0 0 0>;
+               compatible = "intel,irq-router";
+               intel,pirq-config = "pci";
+               intel,pirq-link = <0x60 8>;
+               intel,pirq-mask = <0xdef8>;
+               intel,pirq-routing = <
+                       PCI_BDF(0, 2, 0) INTA PIRQA
+                       PCI_BDF(0, 3, 0) INTA PIRQB
+                       PCI_BDF(0, 8, 0) INTA PIRQC
+                       PCI_BDF(0, 8, 1) INTB PIRQD
+                       PCI_BDF(1, 6, 0) INTA PIRQE
+                       PCI_BDF(1, 6, 1) INTB PIRQF
+                       PCI_BDF(1, 6, 2) INTC PIRQG
+                       PCI_BDF(1, 6, 3) INTD PIRQH
+               >;
+       };