optee: provide optee_do_bottom_half() as a common function

Provides optee_do_bottom_half() and optee_stop_async_notif() as common
functions callable from the FF-A ABI part of the driver too.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>