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>