core: Add BSD 2-Clause license to hypercall headers

core: Add BSD 2-Clause license to hypercall headers

This allows to use our types, inline functions etc. for interacting with
the hypervisor from within differently licensed cells.

Contributions came from Valentine, Jean-Philippe, Henning and me. I'm
signing off for Henning as well in the name of Siemens.

CC: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
CC: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>