KEYS: trusted: tee: Make registered shm dependency explicit

TEE trusted keys support depends on registered shared memory support
since the key buffers are needed to be registered with OP-TEE. So make
that dependency explicit to not register trusted keys support if
underlying implementation doesn't support registered shared memory.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>