MAINTAINERS: add entries for tee-related orphaned files

Add orphaned TEE/OP-TEE-related files to TEE subsystem entry in
MAINTAINERS. This includes:
- optee_rpmb cmd and test for it
- Misc. OP-TEE tests
- OP-TEE SCMI agent implementation
- Documentation, including device tree bindings

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