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_suspend.c b/lib/psci/psci_suspend.c
index ffe3a91..f71994d 100644
--- a/lib/psci/psci_suspend.c
+++ b/lib/psci/psci_suspend.c
@@ -124,7 +124,7 @@
 	 * TODO : Introduce a mechanism to query the cache level to flush
 	 * and the cpu-ops power down to perform from the platform.
 	 */
-	psci_do_pwrdown_sequence(max_off_lvl);
+	psci_pwrdown_cpu(max_off_lvl);
 
 #if ENABLE_RUNTIME_INSTRUMENTATION
 	PMF_CAPTURE_TIMESTAMP(rt_instr_svc,