dt-bindings: qdsp: device-tree documentation for fastrpc nodes

Device-tree documentation for the fastrpc nodes which contain
properties and IOMMU info for the various context banks.

Change-Id: Ic5c28f1f82d8cf631555115feaca100d5a0d4a1c
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>