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>
diff --git a/lib/psci/psci_off.c b/lib/psci/psci_off.c
index 5447045..637adb9 100644
--- a/lib/psci/psci_off.c
+++ b/lib/psci/psci_off.c
@@ -109,7 +109,7 @@
 	/*
 	 * Arch. management. Initiate power down sequence.
 	 */
-	psci_do_pwrdown_sequence(psci_find_max_off_lvl(&state_info));
+	psci_pwrdown_cpu(psci_find_max_off_lvl(&state_info));
 
 #if ENABLE_RUNTIME_INSTRUMENTATION
 	PMF_CAPTURE_TIMESTAMP(rt_instr_svc,