plat: marvell: armada: a8k: common: Fix a8k_common.mk to use BOARD_DIR variable

Use the BOARD_DIR variable instead of PLAT_FAMILY_BASE variable for
determening the path of the system_power.c file.

The variable was not updated, when it was deprecated in a8k_common.mk
in commit 613bbde0

.
Signed-off-by: Luka Kovacic <luka.kovacic@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Change-Id: I9b4659a19ba3cd5c869d44c5d834b220f49136e8

plat/marvell/armada/a8k/common/a8k_common.mk

@@ -37,7 +37,7 @@ ROM_BIN_EXT ?= $(BUILD_PLAT)/ble.bin
 DOIMAGE_FLAGS	+= -b $(ROM_BIN_EXT) $(NAND_DOIMAGE_FLAGS) $(DOIMAGE_SEC_FLAGS)
 
 # Check whether to build system_power.c for the platform
-ifneq ("$(wildcard $(PLAT_FAMILY_BASE)/$(PLAT)/board/system_power.c)","")
+ifneq ("$(wildcard $(BOARD_DIR)/board/system_power.c)","")
 SYSTEM_POWER_SUPPORT = 1
 else
 SYSTEM_POWER_SUPPORT = 0
@@ -90,7 +90,7 @@ MARVELL_DRV		:= 	$(MARVELL_DRV_BASE)/io_win.c	\
 BL31_PORTING_SOURCES	:=	$(BOARD_DIR)/board/marvell_plat_config.c
 
 ifeq ($(SYSTEM_POWER_SUPPORT),1)
-BL31_PORTING_SOURCES	+=	$(PLAT_FAMILY_BASE)/$(PLAT)/board/system_power.c
+BL31_PORTING_SOURCES	+=	$(BOARD_DIR)/board/system_power.c
 endif
 
 BL31_SOURCES		+=	lib/cpus/aarch64/cortex_a72.S		       \