refactor(st): move plat_image_load.c

This file is common for ST platforms, move it to plat/st/common/.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I7cefbc7f857d4ea63320042988c86d28e8a3cc09
diff --git a/plat/st/common/common.mk b/plat/st/common/common.mk
index f69c901..d10f185 100644
--- a/plat/st/common/common.mk
+++ b/plat/st/common/common.mk
@@ -146,6 +146,7 @@
 
 BL2_SOURCES			+=	drivers/io/io_fip.c				\
 					plat/st/common/bl2_io_storage.c			\
+					plat/st/common/plat_image_load.c		\
 					plat/st/common/stm32mp_fconf_io.c
 
 BL2_SOURCES			+=	drivers/io/io_block.c				\