OvmfPkg: install an SSDT with a dynamic OperationRegion called FWDT

OvmfPkg: install an SSDT with a dynamic OperationRegion called FWDT

"FWDT" ("firmware data") is allocated as EfiReservedMemoryType, with
AllocateReservedPool(). <MdePkg/Include/Library/MemoryAllocationLib.h>
doesn't seem to provide direct access to EfiACPIReclaimMemory, but at this
point the former seems sufficient.

Based on SeaBIOS commit 2062f2ba by Gerd Hoffmann <kraxel@redhat.com>.

v3:
- coding style fixes:
  - BDAT     -> FWDT
  - __packed -> #pragma pack(1)
  - BFLD     -> FIRMWARE_DATA, PCI_WINDOW
  - Bfld     -> FwData
- Ssdt.asl: changed license to 2-clause BSDL, paraphrasing Dsdt.asl

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13573 6f19259b-4bc3-4df7-8a09-765794883524