plat-sam: force CFG_EXTERNAL_DT to n

Because this feature isn't used on SAM platforms and to disable DT
insecure warning.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/arm/plat-sam/conf.mk

@@ -39,6 +39,9 @@ $(call force,CFG_CORE_HAS_GENERIC_TIMER,n)
 $(call force,CFG_TZDRAM_START,0x20000000)
 $(call force,CFG_TZDRAM_SIZE,0x800000)
 
+# This value is forced because this feature isn't used by SAM platforms.
+$(call force,CFG_EXTERNAL_DT,n)
+
 CFG_MMAP_REGIONS ?= 24
 
 CFG_SHMEM_START  ?= 0x21000000