iommu/omap: Add support for configuring dsp iommus on DRA7xx

The DSP MMUs on DRA7xx SoC requires configuring an additional
MMU_CONFIG register present in the DSP_SYSTEM sub module. This
setting dictates whether the DSP Core's MDMA and EDMA traffic
is routed through the respective MMU or not. Add the support
to the OMAP iommu driver so that the traffic is not bypassed
when enabling the MMUs.

The MMU_CONFIG register has two different bits for enabling
each of these two MMUs present in the DSP processor sub-system
on DRA7xx. An id field is added to the OMAP iommu object to
identify and enable each IOMMU. The id information and the
DSP_SYSTEM.MMU_CONFIG register programming is achieved through
the processing of the optional "ti,syscon-mmuconfig" property.
A proper value is assigned to the id field only when this
property is present.

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