ArmVirtPkg: drop dependency on PeiPcdLib for PEI Pcd.inf

ArmVirtPkg: drop dependency on PeiPcdLib for PEI Pcd.inf

The PcdPeim dynamic PCD driver is dispatched explicitly via an 'A PRIORI'
declaration in the platform DSC. Without that declaration, the PEI module
can never be dispatched since it transitively (via PeiPcdLib) depends on
a PPI 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>