cmd: remoteproc: Add support for initializing devices individually

cmd: remoteproc: Add support for initializing devices individually

'rproc init' does the probe and initialization of all the available
remoteproc devices in the system. This doesn't allow the flexibility
to initialize the remote cores needed as per use case. In order
to provide flexibility, update 'rproc init' command to accept one
more parameter with rproc id which when passed initializes only
that specific core. If no id is passed, command will initializes
all the cores which is compatible with the existing behaviour.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>