ARM: dts: keystone-k2g: Add DSP node

The Keystone 2 66AK2G SoC has a single TMS320C66x DSP Core
Subsystem (C66x CorePac), containing a C66x Fixed/Floating-Point
DSP Core, and 32 KB of L1P & L1D SRAMs and a 1 MB L2 SRAM. Add
the DT node for this DSP processor sub-system.

The DT node has a new property 'power-domains' and no 'clocks'
properties, and uses slightly different property values for
'resets' compared to other Keystone 2 SoCs. The processor does
not have an MMU, and uses various IPC Generation registers and
shared memory for inter-processor communication. The alias with
a stem 'rproc' has also been added for the DSP, it provides a
fixed remoteproc id for the DSP processor.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>