tee: optee: GET_OS_REVISION: document a2 as a build identifier

tee: optee: GET_OS_REVISION: document a2 as a build identifier

In the OPTEE_SMC_CALL_GET_OS_REVISION request, the previously reserved
parameter a2 is now documented as being an optional build identifier
(such as an SCM revision or commit ID, for instance).

A new structure optee_smc_call_get_os_revision_result is introduced to
be used when querying the secure OS version, instead of re-using the
struct defined for OPTEE_SMC_CALLS_REVISION.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Matthias Brugger <mbruger@suse.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>