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>