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: f74cbc93a ("stm32mp1: Link BL2, BL32 and DTB in one binary")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>