[sw/silicon_creator] Add a constant for the UNIMP instruction used for padding
[sw/silicon_creator] Add a constant for the UNIMP instruction used for padding

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

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