refactor(psci): move psci_do_pwrdown_sequence() out of private header

Move the psci_do_pwrdown_sequence() function declaration from PSCI
private header to common header. The psci_do_pwrdown_sequence is
required to support warm reset, where each CPU need to execute the
powerdown sequence.

Change-Id: I298e7a120be814941fa91c0b001002a080e56263
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>