Documentation: dt: Update OMAP iommu bindings for DRA7 DSPs

The DSP processor sub-systems on DRA7xx have two MMU instances
each, one for the processor core and the other for an internal
EDMA block. These MMUs need an additional shared register to be
programmed in the DSP_SYSTEM sub-module to be enabled properly.

The OMAP IOMMU bindings is updated to account for this additional
syscon property required for these DSP IOMMU instances on DRA7xx
SoCs. A new compatible "ti,dra7-dsp-iommu" is also defined to
distinguish these devices specifically from other DRA7 IOMMU
devices.

An example of the DRA7 DSP IOMMU nodes is also added to the
document for clarity.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>