ArmVirtPkg: VirtFdtDxe: detect fw-cfg DMA interface from the DTB

ArmVirtPkg: VirtFdtDxe: detect fw-cfg DMA interface from the DTB

A DMA-like transfer interface has recently been implemented in QEMU for
fw-cfg. For ARM and AARCH64 virtual machines, the binding prescribes a new
8-byte wide register at offset 0x10 in the register block. Make VirtFdtDxe
expose this register if it is present.

Please see "docs/specs/fw_cfg.txt" in the QEMU tree for more information.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18544 6f19259b-4bc3-4df7-8a09-765794883524