Documentation: dt: add OMAP iommu bindings

This patch adds the iommu bindings for all OMAP2+ SoCs. Apart from
the standard bindings used by OMAP peripherals, this patch uses a
'dma-window' (already used by Tegra SMMU) and adds two OMAP custom
bindings - 'ti,#tlb-entries' and 'ti,iommu-bus-err-back'.
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
[s-anna@ti.com: split bindings document, add dra7 and bus error back]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>

Documentation/devicetree/bindings/iommu/ti,omap-iommu.txt

+OMAP2+ IOMMU
+
+Required properties:
+- compatible : Should be one of,
+		"ti,omap2-iommu" for OMAP2/OMAP3 IOMMU instances
+		"ti,omap4-iommu" for OMAP4/OMAP5 IOMMU instances
+		"ti,dra7-iommu" for DRA7xx IOMMU instances
+- ti,hwmods  : Name of the hwmod associated with the IOMMU instance
+- reg        : Address space for the configuration registers
+- interrupts : Interrupt specifier for the IOMMU instance
+
+Optional properties:
+- ti,#tlb-entries : Number of entries in the translation look-aside buffer.
+                    Should be either 8 or 32 (default: 32)
+- ti,iommu-bus-err-back : Indicates the IOMMU instance supports throwing
+		          back a bus error response on MMU faults.
+
+Example:
+	/* OMAP3 ISP MMU */
+	mmu_isp: mmu@480bd400 {
+		compatible = "ti,omap2-iommu";
+		reg = <0x480bd400 0x80>;
+		interrupts = <24>;
+		ti,hwmods = "mmu_isp";
+		ti,#tlb-entries = <8>;
+	};