ArmVirtPkg/ArmGicArchLib: move to FdtClient protocol

ArmVirtPkg/ArmGicArchLib: move to FdtClient protocol

Instead of relying on VirtFdtDxe to populate the GIC related PCDs, move
this handling to our implementation of ArmGicArchLib, and retrieve the
required DT info using the new FDT client protocol.

This removes one of the reasons we need to load VirtFdtDxe first using
an 'A PRIORI' declaration in the platform FDF.

As Laszlo kindly confirms:

  So, ultimately, the only user of this library instance is
  "ArmPkg/Drivers/ArmGic/ArmGicDxe.inf". ... Indeed, checking the build
  report file for ArmVirtQemu (AARCH64), I find ArmVirtGicArchLib (and
  ArmGicLib too) only under "ArmPkg/Drivers/ArmGic/ArmGicDxe.inf".

which means that the constructor is only invoked once, and so the dynamic
PCDs are set in time for ArmGicDxe to consume them, and never afterwards.

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