ArmPlatformPkg/ArmVExpressDxe: remove FDT handling from ArmFvpDxe

ArmPlatformPkg/ArmVExpressDxe: remove FDT handling from ArmFvpDxe

Replace the elaborate but awkward handling of FDT images using device
paths and string PCDs initialized to 128 spaces with a simple scheme
involving a set of builtin DTBs and a bit of runtime logic to select
between them.

This is sufficient for ordinary use, which makes it more suitable as
reference code. Note that overriding the DTB presented to the OS can
easily be done with a UEFI application that simply installs a new DTB
image under the existing FDT configuration table GUID.

For this module, this simply involves removing all code that is involved
in deciding which platform we are running on, and for reasoning about
FDT device paths.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>