arm64: dts: ti: k3-j721e-main: Add C71x DSP node

The J721E SoCs have a single TMS320C71x DSP Subsystem in the MAIN
voltage domain containing the next-generation C711 CPU core. The
subsystem has 32 KB of L1D configurable SRAM/Cache and 512 KB of
L2 configurable SRAM/Cache. This subsystem has a CMMU but is not
used currently. The inter-processor communication between the main
A72 cores and the C711 processor is achieved through shared memory
and a Mailbox. Add the DT node for this DSP processor sub-system
in the common k3-j721e-main.dtsi file.

The following firmware name is used by default for the C71x core,
and can be overridden in a board dts file if desired:
    C71x_0 DSP: j7-c71_0-fw

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20200825172145.13186-6-s-anna@ti.com