ARM: dts: DRA7: Add dsp1_system syscon node

The DSP_SYSTEM sub-module is a dedicated system control logic
module present within a DRA7 DSP processor sub-system. This
module is responsible for power management, clock generation
and connection to the device PRCM module.

Add a syscon node for this module for the DSP1 processor
sub-system. This is added as a syscon node as it is a common
configuration module that can be used by the different IOMMU
instances and the corresponding remoteproc device.

The node is added to the common dra7.dtsi file, as the DSP1
processor sub-system is mostly common across all the variants
of the DRA7 SoC family.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>