[TF-A] Allow platforms to deny CPU power off request

Hello,

We have a use case where we want to deny the CPU power off from a certain PE. The current lib/psci does not provide a way for a platform to do this. We have support to return an error from svc_off, but this does not cover the scenario where svc_off is NULL.

There are multiple ways of handling this scenario.

1. Introduce a new platform function to check if CPU_OFF is allowed on the core 2. Return an error code from the pwr_domain_off() handler and unwind the CPU_OFF request 3. Something else.

Any suggestions?

-Varun