ArmVirtualizationPkg: VirtFdtDxe: forward FwCfg addresses from DTB to PCDs
    
    Qemu's firmware configuration interface consists of two MMIO registers, a
    16-bit selector and an 8-bit data register. Parse their addresses and
    verify their sizes from the DTB, and expose them to the rest of DXE by
    storing them in dynamic PCDs.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>