iommu/ipmmu-vmsa: Add device tree bindings documentation

Signed-off-by: NLaurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>

Documentation/devicetree/bindings/iommu/renesas,ipmmu-vmsa.txt

+* Renesas VMSA-Compatible IOMMU
+
+The IPMMU is an IOMMU implementation compatible with the ARM VMSA page tables.
+It provides address translation for bus masters outside of the CPU, each
+connected to the IPMMU through a port called micro-TLB.
+
+
+Required Properties:
+
+  - compatible: Must contain "renesas,ipmmu-vmsa".
+  - reg: Base address and size of the IPMMU registers.
+  - interrupts: Specifiers for the MMU fault interrupts. For instances that
+    support secure mode two interrupts must be specified, for non-secure and
+    secure mode, in that order. For instances that don't support secure mode a
+    single interrupt must be specified.
+
+  - #iommu-cells: Must be 1.
+
+Each bus master connected to an IPMMU must reference the IPMMU in its device
+node with the following property:
+
+  - iommus: A reference to the IPMMU in two cells. The first cell is a phandle
+    to the IPMMU and the second cell the number of the micro-TLB that the
+    device is connected to.
+
+
+Example: R8A7791 IPMMU-MX and VSP1-D0 bus master
+
+	ipmmu_mx: mmu@fe951000 {
+		compatible = "renasas,ipmmu-vmsa";
+		reg = <0 0xfe951000 0 0x1000>;
+		interrupts = <0 222 IRQ_TYPE_LEVEL_HIGH>,
+			     <0 221 IRQ_TYPE_LEVEL_HIGH>;
+		#iommu-cells = <1>;
+	};
+
+	vsp1@fe928000 {
+		...
+		iommus = <&ipmmu_mx 13>;
+		...
+	};