ArmVirtualizationPkg: VirtFdtDxe: forward FwCfg addresses from DTB to PCDs

ArmVirtualizationPkg: VirtFdtDxe: forward FwCfg addresses from DTB to PCDs

Qemu's firmware configuration interface for ARM consists of two MMIO
registers, a 16-bit selector, and a 64-bit data register that allows the
guest to transfer data with 8, 16, 32, and 64-bit wide accesses. Parse the
base address from the DTB, and expose the registers to the rest of DXE via
dynamic PCDs.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16566 6f19259b-4bc3-4df7-8a09-765794883524