stm32mp1: platform.mk: use PHONY for the appropriate targets

Currently, building TF-A for STM32MP1 triggers a full rebuild,
avoid this by removing the .PHONY: specification for the final image and
replace it by specifying PHONYness for the targets that don't actually
produce file output.

This will come in handy in follow-up commits, when implicit rules are
introduced, as implicit rule search is skipped for .PHONY targets.

Change-Id: Ib9966479032b081a54123b99f889760e85639f19
Fixes: f74cbc93

 ("stm32mp1: Link BL2, BL32 and DTB in one binary")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

plat/st/stm32mp1/platform.mk

@@ -164,7 +164,7 @@ BL2_CFLAGS	+=	-DPLAT_XLAT_TABLES_DYNAMIC=1
 STM32IMAGEPATH		?= tools/stm32image
 STM32IMAGE		?= ${STM32IMAGEPATH}/stm32image${BIN_EXT}
 
-.PHONY:			${STM32_TF_STM32}
+.PHONY:			check_dtc_version stm32image clean_stm32image
 .SUFFIXES:
 
 all: check_dtc_version ${STM32_TF_STM32} stm32image