dm crypt: support using trusted keys

Commit 27f5411a ("dm crypt: support using encrypted keys") extended
dm-crypt to allow use of "encrypted" keys along with "user" and "logon".

Along the same lines, teach dm-crypt to support "trusted" keys as well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>