fw_cfg: add a public uapi header

Create a common header file for well-known values and structures to be
shared by the Linux kernel with qemu or other projects.

It is based from qemu/docs/specs/fw_cfg.txt which references
qemu/include/hw/nvram/fw_cfg_keys.h "for the most up-to-date and
authoritative list" & vmcoreinfo.txt. Those files don't have an
explicit license, but qemu/hw/nvram/fw_cfg.c is BSD-license, so
Michael S. Tsirkin suggested to use the same license.

The patch intentionally left out DMA & vmcoreinfo structures &
defines, which are added in the commits making usage of it.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>