Include `plat_psci_common.c` from the new location

The `plat_psci_common.c` was moved to the new location `plat/common`
and a stub file was retained at previous location for compatibility. This
patch modifies the platform makefiles to include the file from the new
location.

Change-Id: Iabddeeb824e9a5d72d176d7c644735966c8c0699
diff --git a/plat/arm/common/arm_common.mk b/plat/arm/common/arm_common.mk
index 9e5ddea..249a406 100644
--- a/plat/arm/common/arm_common.mk
+++ b/plat/arm/common/arm_common.mk
@@ -128,7 +128,7 @@
 				plat/arm/common/arm_pm.c			\
 				plat/arm/common/arm_topology.c			\
 				plat/common/aarch64/platform_mp_stack.S		\
-				plat/common/aarch64/plat_psci_common.c
+				plat/common/plat_psci_common.c
 
 ifneq (${TRUSTED_BOARD_BOOT},0)