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/include/lib/psci/psci_lib.h b/include/lib/psci/psci_lib.h
index 43e2f96..3edc50b 100644
--- a/include/lib/psci/psci_lib.h
+++ b/include/lib/psci/psci_lib.h
@@ -92,6 +92,7 @@
 int psci_stop_other_cores(unsigned int wait_ms,
 			  void (*stop_func)(u_register_t mpidr));
 bool psci_is_last_on_cpu_safe(void);
+void psci_pwrdown_cpu(unsigned int power_level);
 
 #endif /* __ASSEMBLER__ */