MdePkg: BasePciLibPciExpress: list ARM and AARCH64 as valid architectures
"ArmPlatformPkg/ArmVirtualizationPkg/ArmVirtualization.dsc.inc" references
this PciLib instance.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>