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_common.c b/lib/psci/psci_common.c
index b60ddbb..efcfed8 100644
--- a/lib/psci/psci_common.c
+++ b/lib/psci/psci_common.c
@@ -954,7 +954,7 @@
  * Initiate power down sequence, by calling power down operations registered for
  * this CPU.
  ******************************************************************************/
-void psci_do_pwrdown_sequence(unsigned int power_level)
+void psci_pwrdown_cpu(unsigned int power_level)
 {
 #if HW_ASSISTED_COHERENCY
 	/*