OvmfPkg: Add PCD for Host Bridge dev. ID (PcdOvmfHostBridgePciDevId)

Set from PEI, this PCD allows subsequent stages (specifically
DXE_DRIVER and DXE_RUNTIME_DRIVER) to infer the underlying platform
type (e.g. PIIX4 or Q35/MCH) without the need to further query the
Host Bridge for its Device ID.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16374 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/PlatformPei/Platform.c

@@ -245,7 +245,7 @@ MiscInitialization (
   BuildCpuHob (36, 16);
 
   //
-  // Query Host Bridge DID to determine platform type
+  // Query Host Bridge DID to determine platform type and save to PCD
   //
   HostBridgeDevId = PciRead16 (OVMF_HOSTBRIDGE_DID);
   switch (HostBridgeDevId) {
@@ -265,6 +265,7 @@ MiscInitialization (
       ASSERT (FALSE);
       return;
   }
+  PcdSet16 (PcdOvmfHostBridgePciDevId, HostBridgeDevId);
 
   //
   // If PMREGMISC/PMIOSE is set, assume the ACPI PMBA has been configured (for