refactor: panic after calling psci_power_down_wfi()

This function doesn't return and its callers that don't return either
rely on this. Drop the dead attribute and add a panic() after it to make
this expectation explicit. Calling `wfi` in the powerdown sequence is
terminal so even if the function was made to return, there would be no
functional change.

This is useful for a following patch that makes psci_power_down_wfi()
return.

Change-Id: I62ca1ee058b1eaeb046966c795081e01bf45a2eb
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
diff --git a/plat/allwinner/common/sunxi_native_pm.c b/plat/allwinner/common/sunxi_native_pm.c
index 148f50e..558b0bb 100644
--- a/plat/allwinner/common/sunxi_native_pm.c
+++ b/plat/allwinner/common/sunxi_native_pm.c
@@ -49,6 +49,8 @@
 	sunxi_cpu_power_off_others();
 	sunxi_cpu_power_off_self();
 	psci_power_down_wfi();
+	/* should never reach here */
+	panic();
 }
 
 static void __dead2 sunxi_system_reset(void)