ArmVirtPkg: implement FdtPciPcdProducerLib

ArmVirtPkg: implement FdtPciPcdProducerLib

This implements a library FdtPciPcdProducerLib which is intended to
be incorporated into modules that consume the PCI related dynamic PCDs
PcdPciExpressBaseAddress and PcdPciDisableBusEnumeration, either via NULL
library class resolution or via a direct dependency (for other libraries
or modules in ArmVirtPkg). This allows us to make them depend on the FDT
client protocol, and populate these PCDs based on the presence and the
contents of a 'pci-host-ecam-generic' DT node.

This also overloads the meaning of PcdPciExpressBaseAddress, which we will
set to MAX_UINT64 to signify that the actual values of these two PCDs have
not been assigned yet.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>