NFIT is defined in ACPI 6.0: 5.2.25 NVDIMM Firmware Interface Table (NFIT)

Currently, we only support PMEM mode. Each device has 3 structures:
- SPA structure, defines the PMEM region info

- MEM DEV structure, it has the @handle which is used to associate specified
  ACPI NVDIMM  device we will introduce in later patch.
  Also we can happily ignored the memory device's interleave, the real
  nvdimm hardware access is hidden behind host

- DCR structure, it defines vendor ID used to associate specified vendor
  nvdimm driver. Since we only implement PMEM mode this time, Command
  window and Data window are not needed

The NVDIMM functionality is controlled by the parameter, 'nvdimm', which
is introduced for the machine, there is a example to enable it:
-machine pc,nvdimm -m 8G,maxmem=100G,slots=100  -object \
memory-backend-file,id=mem1,share,mem-path=/tmp/nvdimm1,size=10G -device \
nvdimm,memdev=mem1,id=nv1

It is disabled on default
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NXiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>