optee: use export_uuid() to copy client UUID

Prior to this patch optee_open_session() was making assumptions about
the internal format of uuid_t by casting a memory location in a
parameter struct to uuid_t *. Fix this using export_uuid() to get a well
defined binary representation and also add an octets field in struct
optee_msg_param in order to avoid casting.

Fixes: c5b4312b

 ("tee: optee: Add support for session login client UUID generation")
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>