xen/arm: optee: add OP-TEE header files

xen/arm: optee: add OP-TEE header files

This header files describes protocol between OP-TEE OS and OP-TEE
clients, which are running in Normal World. This headers are needed
for upcoming OP-TEE mediator, which is added in the next patch. Reason
to add those headers in separate patch is to ease up review. Those
files were taken from OP-TEE OS 3.5.0 tree and mangled a bit to
compile with XEN.

Location of the files in the original tree:

core/include/optee_msg.h
core/include/optee_rpc_cmd.h
core/arch/arm/include/sm/optee_smc.h

Note the imported header is licensed BSD-2-clause. This is fine as it is
compatible with GPLv2-only.

[OP-TEE commit id 5df2a985b2ffd0b6f1107f12ca2a88203bf31328]

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
[julien: Add a sentence regarding the license]
Acked-by: Julien Grall <julien.grall@arm.com>