ArmVirtPkg: drop dependency on DxePcdLib for DXE Pcd.inf

ArmVirtPkg: drop dependency on DxePcdLib for DXE Pcd.inf

The PcdDxe dynamic PCD driver is dispatched explicitly via an 'A PRIORI'
declaration in the platform DSC. Without that declaration, the DXE driver
can never be dispatched since it transitively (via DxePcdLib) depends on
protocols it produces itself. So use the NULL PcdLib explicitly only for
this driver.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>