refactor(plat/st): avoid fixed DT address

Device Tree address is now a parameter for dt_open_and_check() function.
This will allow better flexibility when introducing PIE and FIP.
The fdt pointer is now only assigned if the given address holds
a valid device tree file. This allows removing the fdt_checked variable,
as we now check fdt is not null.

Change-Id: I04cbb2fc05c9c711ae1c77d56368dbeb6dd4b01a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/plat/st/stm32mp1/bl2_plat_setup.c b/plat/st/stm32mp1/bl2_plat_setup.c
index 0e95f49..91073b8 100644
--- a/plat/st/stm32mp1/bl2_plat_setup.c
+++ b/plat/st/stm32mp1/bl2_plat_setup.c
@@ -196,7 +196,7 @@
 
 	configure_mmu();
 
-	if (dt_open_and_check() < 0) {
+	if (dt_open_and_check(STM32MP_DTB_BASE) < 0) {
 		panic();
 	}