Move `arm_common.c` out of aarch64 folder

This patch moves the `arm_common.c` file from `plat/arm/common/aarch64/`
to the parent directory since the functions implemented in the file are
not AArch64 specific. The platform makefiles are also modified for this
change.

Change-Id: I776d2e4958f59041476cf2f53a9adb5b2d304ee0

plat/arm/common/arm_common.mk

@@ -97,8 +97,8 @@ PLAT_INCLUDES		+=	-Iinclude/common/tbbr				\
 
 PLAT_BL_COMMON_SOURCES	+=	lib/xlat_tables/xlat_tables_common.c		\
 				lib/xlat_tables/aarch64/xlat_tables.c		\
-				plat/arm/common/aarch64/arm_common.c		\
 				plat/arm/common/aarch64/arm_helpers.S		\
+				plat/arm/common/arm_common.c			\
 				plat/common/aarch64/plat_common.c
 
 BL1_SOURCES		+=	drivers/arm/sp805/sp805.c			\

plat/xilinx/zynqmp/platform.mk

@@ -68,9 +68,9 @@ PLAT_BL_COMMON_SOURCES	:=	lib/xlat_tables/xlat_tables_common.c		\
 				drivers/arm/gic/v2/gicv2_helpers.c		\
 				drivers/cadence/uart/cdns_console.S		\
 				drivers/console/console.S			\
-				plat/arm/common/aarch64/arm_common.c		\
 				plat/arm/common/aarch64/arm_helpers.S		\
 				plat/arm/common/arm_cci.c			\
+				plat/arm/common/arm_common.c			\
 				plat/arm/common/arm_gicv2.c			\
 				plat/common/plat_gicv2.c			\
 				plat/common/aarch64/plat_common.c		\