optee: support asynchronous supplicant requests

Adds support for asynchronous supplicant requests, meaning that the
supplicant can process several requests in parallel or block in a
request for some time.

Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Tested-by: Etienne Carriere <etienne.carriere@linaro.org> (b2260 pager=y/n)
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>