MdeModulePkg: introduce PcdPciDisableBusEnumeration

Platforms such as Xen already enumerates PCI bridges and devices. Use
this PCD to control EDK2 behavior.

PcdPciDisableBusEnumeration is placed under [PcdsFixedAtBuild,
PcdsPatchableInModule, PcdsDynamic, PcdsDynamicEx], so that it is
possible to alter it during runtime.

The default setting of this PCD is false (allow full PCI enumeration) to
preserve the same behavior before this change.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@14940 6f19259b-4bc3-4df7-8a09-765794883524