ArmPlatformPkg/ArmVExpressDxe: Change FDT default file names.

ArmPlatformPkg/ArmVExpressDxe: Change FDT default file names.

On the FVP base and foundation models, the default file name
used to retrieve the FDT depended on the values assigned to
model parameters (GIC related model parameters).

Now, in addition to the fallback "fdt.dtb" file name (used for legacy
 reason), only one default file name is used :
- "fvp-base.dtb" for the base model.
- "fvp-foundation.dtb" for the foundation model.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Ronald Cron <Ronald.Cron@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17862 6f19259b-4bc3-4df7-8a09-765794883524