cmd: optee_rpmb: close tee session

Close tee session after each optee_rpmb invocation, as there is no
reason to keep it open, considering the absence of any available mechanism
to clean up all open sessions automatically before handing over control
to the Linux kernel. Without proper clean-up we might end up with orphaned
sessions registered in OP-TEE OS core (obvious resource leak).

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>