[aes] Only unmask final cipher core output
[aes] Only unmask final cipher core output

This commit inserts a mux between the masked cipher core output and the
data output register/feedback path to:
1. Avoid SCA leakage occuring from unmasking intermediate cipher core
   outputs.
2. Increase the difficulty for extracting intermediate data via FI.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan