This function doesn't return and its callers that don't return either
rely on this. Drop the dead attribute and add a panic() after it to make
this expectation explicit. Calling `wfi` in the powerdown sequence is
terminal so even if the function was made to return, there would be no
functional change.
This is useful for a following patch that makes psci_power_down_wfi()
return.
Change-Id: I62ca1ee058b1eaeb046966c795081e01bf45a2eb
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
