arm64: dts: ti: k3-j721e: Add an alias for C71x rproc node

Add the alias for the C71x remoteproc processor core present
within the MAIN domain and common to all the K3 J721E boards.
The alias uses the stem "rproc".

The rproc aliases can be overridden, if needed, in the respective
derivative board dts files.

Signed-off-by: Suman Anna <s-anna@ti.com>