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>

ArmVirtPkg/ArmVirtQemu.dsc

@@ -255,7 +255,10 @@
   #
   ArmPlatformPkg/PrePeiCore/PrePeiCoreUniCore.inf
   MdeModulePkg/Core/Pei/PeiMain.inf
-  MdeModulePkg/Universal/PCD/Pei/Pcd.inf
+  MdeModulePkg/Universal/PCD/Pei/Pcd.inf {
+    <LibraryClasses>
+      PcdLib|MdePkg/Library/BasePcdLibNull/BasePcdLibNull.inf
+  }
   ArmPlatformPkg/PlatformPei/PlatformPeim.inf
   ArmPlatformPkg/MemoryInitPei/MemoryInitPeim.inf
   ArmPkg/Drivers/CpuPei/CpuPei.inf

ArmVirtPkg/ArmVirtQemu.fdf

@@ -258,9 +258,6 @@ READ_STATUS        = TRUE
 READ_LOCK_CAP      = TRUE
 READ_LOCK_STATUS   = TRUE
 
-  APRIORI PEI {
-    INF  MdeModulePkg/Universal/PCD/Pei/Pcd.inf
-  }
   INF ArmPlatformPkg/PrePeiCore/PrePeiCoreUniCore.inf
   INF MdeModulePkg/Core/Pei/PeiMain.inf
   INF ArmPlatformPkg/PlatformPei/PlatformPeim.inf