xen/arm: optee: add OP-TEE mediator skeleton

xen/arm: optee: add OP-TEE mediator skeleton

Add very basic OP-TEE mediator. It can probe for OP-TEE presence,
tell it about domain creation/destruction and then return an error
to all calls to the guest.

This code issues two non-preemptible calls to OP-TEE: to create and
to destroy client context. They can't block in OP-TEE, as they are
considered "fast calls" in terms of ARM SMCCC.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Julien Grall <julien.grall@arm.com>