remoteproc: tisci_proc: Add helper api for controlling core power domain

Power domain for the remote cores needs to be handled in a right
sequence as mandated by the spec. Introduce tisci helper apis
that can control power-domains of remote cores. TISCI clients
can use this api and control the remote cores power domain instead
of hooking it to power-domain layer.

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