vexpress64: Use OF_HAS_PRIOR_STAGE for BASE_FVP variant

BASE_FVP now typically uses a devicetree provided by a prior boot stage
(typically Arm TF-A), so imply this option by default when
TARGET_VEXPRESS64_BASE_FVP is selected.

OF_HAS_PRIOR_STAGE selects OF_BOARD so this change is minor, but aligns
TARGET_VEXPRESS64_BASE_FVP with TARGET_VEXPRESS64_BASER_FVP.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/board/armltd/vexpress64/Kconfig b/board/armltd/vexpress64/Kconfig
index 5616e22..bd15cbe 100644
--- a/board/armltd/vexpress64/Kconfig
+++ b/board/armltd/vexpress64/Kconfig
@@ -23,7 +23,7 @@
 config TARGET_VEXPRESS64_BASE_FVP
 	bool "Support Versatile Express ARMv8a FVP BASE model"
 	select VEXPRESS64_BASE_MODEL
-	select OF_BOARD
+	imply OF_HAS_PRIOR_STAGE
 
 config TARGET_VEXPRESS64_BASER_FVP
 	bool "Support Versatile Express ARMv8r64 FVP BASE model"