ti: k3: drivers: ti_sci: Use non-blocking TI-SCI messages for power down

Now that we have non-blocking TI-SCI functions we can initiate the shutdown
sequence from the PSCI handler without needing the ti_sci_proc_shutdown
helper function, which is removed. This gives us the greater control and
flexibility that will be needed when cluster power down sequences are added.

Signed-off-by: Andrew F. Davis <afd@ti.com>