dmaengine: edma: Add TI EDMA device tree binding

The binding definition is based on the generic DMA controller
binding.

Joel:
* Droped reserved and queue DT entries from Documentation
for now from the original patch series (v10)
* Included properties in Documentation and clarified DMA properties (V11)
* Made ti,hwmod option
* Clarified DMA entries
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Matt Porter <mporter@ti.com>
Signed-off-by: Joel A Fernandes <joelagnel@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>

Documentation/devicetree/bindings/dma/ti-edma.txt

+TI EDMA
+
+Required properties:
+- compatible : "ti,edma3"
+- ti,edma-regions: Number of regions
+- ti,edma-slots: Number of slots
+- #dma-cells: Should be set to <1>
+              Clients should use a single channel number per DMA request.
+- dma-channels: Specify total DMA channels per CC
+- reg: Memory map for accessing module
+- interrupt-parent: Interrupt controller the interrupt is routed through
+- interrupts: Exactly 3 interrupts need to be specified in the order:
+              1. Transfer completion interrupt.
+              2. Memory protection interrupt.
+              3. Error interrupt.
+Optional properties:
+- ti,hwmods: Name of the hwmods associated to the EDMA
+- ti,edma-xbar-event-map: Crossbar event to channel map
+
+Example:
+
+edma: edma@49000000 {
+	reg = <0x49000000 0x10000>;
+	interrupt-parent = <&intc>;
+	interrupts = <12 13 14>;
+	compatible = "ti,edma3";
+	ti,hwmods = "tpcc", "tptc0", "tptc1", "tptc2";
+	#dma-cells = <1>;
+	dma-channels = <64>;
+	ti,edma-regions = <4>;
+	ti,edma-slots = <256>;
+	ti,edma-xbar-event-map = <1 12
+				  2 13>;
+};