ARM: dts: k2g: Add DSP node

The Keystone 2 Galileo SoC has a single TMS320C66x DSP
Core Subsystem (C66x CorePac), containing a C66x Fixed /
Floating-Point DSP Core, and 32KB of L1P & L1D SRAMs and a
1MB L2 SRAM. Add the DT node and the rproc alias for this DSP
processor sub-system.

The DT node has a new property 'power-domains', and uses
slightly different property values for 'clocks' and 'resets'
compared to other Keystone 2 SoCs. The processor does not have
a MMU, and uses various IPC Generation registers and shared
memory for inter-processor communication.

NOTE:
The node name uses a DSP number suffix and does not include the
address for now.  This is required to match the DT name used by
the userland MPM stack for working properly.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>