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_scpi_pm.c b/plat/allwinner/common/sunxi_scpi_pm.c
index 6a0e967..8870a71 100644
--- a/plat/allwinner/common/sunxi_scpi_pm.c
+++ b/plat/allwinner/common/sunxi_scpi_pm.c
@@ -108,6 +108,8 @@
 	}
 
 	psci_power_down_wfi();
+	/* should never reach here */
+	panic();
 }
 
 static void __dead2 sunxi_system_reset(void)
@@ -123,6 +125,8 @@
 	}
 
 	psci_power_down_wfi();
+	/* should never reach here */
+	panic();
 }
 
 static int sunxi_system_reset2(int is_vendor, int reset_type, u_register_t cookie)
@@ -142,6 +146,8 @@
 	}
 
 	psci_power_down_wfi();
+	/* should never reach here */
+	panic();
 
 	/*
 	 * Should not reach here.