firmware: ti_sci: Add processor shutdown API method

Add and expose a new processor shutdown API that wraps the two TISCI
messages involved in initiating a core shutdown. The API will first
queue a message to have the DMSC wait for a certain processor boot
status to happen followed by a message to trigger the actual shutdown-
with both messages being sent without waiting or requesting for a
response. Note that the processor shutdown API call will need to be
followed up by user software placing the respective core into either
WFE or WFI mode.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>