documentation/iommu: Add description of ARM System MMU binding

This patch adds a description of the device tree binding for the ARM
System MMU architecture.

Cc: Rob Herring <robherring2@gmail.com>
Cc: Andreas Herrmann <andreas.herrmann@calxeda.com>
Cc: Joerg Roedel <joro@8bytes.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>

Documentation/devicetree/bindings/iommu/arm,smmu.txt

+* ARM System MMU Architecture Implementation
+
+ARM SoCs may contain an implementation of the ARM System Memory
+Management Unit Architecture, which can be used to provide 1 or 2 stages
+of address translation to bus masters external to the CPU.
+
+The SMMU may also raise interrupts in response to various fault
+conditions.
+
+** System MMU required properties:
+
+- compatible    : Should be one of:
+
+                        "arm,smmu-v1"
+                        "arm,smmu-v2"
+                        "arm,mmu-400"
+                        "arm,mmu-500"
+
+                  depending on the particular implementation and/or the
+                  version of the architecture implemented.
+
+- reg           : Base address and size of the SMMU.
+
+- #global-interrupts : The number of global interrupts exposed by the
+                       device.
+
+- interrupts    : Interrupt list, with the first #global-irqs entries
+                  corresponding to the global interrupts and any
+                  following entries corresponding to context interrupts,
+                  specified in order of their indexing by the SMMU.
+
+                  For SMMUv2 implementations, there must be exactly one
+                  interrupt per context bank. In the case of a single,
+                  combined interrupt, it must be listed multiple times.
+
+- mmu-masters   : A list of phandles to device nodes representing bus
+                  masters for which the SMMU can provide a translation
+                  and their corresponding StreamIDs (see example below).
+                  Each device node linked from this list must have a
+                  "#stream-id-cells" property, indicating the number of
+                  StreamIDs associated with it.
+
+** System MMU optional properties:
+
+- smmu-parent   : When multiple SMMUs are chained together, this
+                  property can be used to provide a phandle to the
+                  parent SMMU (that is the next SMMU on the path going
+                  from the mmu-masters towards memory) node for this
+                  SMMU.
+
+Example:
+
+        smmu {
+                compatible = "arm,smmu-v1";
+                reg = <0xba5e0000 0x10000>;
+                #global-interrupts = <2>;
+                interrupts = <0 32 4>,
+                             <0 33 4>,
+                             <0 34 4>, /* This is the first context interrupt */
+                             <0 35 4>,
+                             <0 36 4>,
+                             <0 37 4>;
+
+                /*
+                 * Two DMA controllers, the first with two StreamIDs (0xd01d
+                 * and 0xd01e) and the second with only one (0xd11c).
+                 */
+                mmu-masters = <&dma0 0xd01d 0xd01e>,
+                              <&dma1 0xd11c>;
+        };