firmware_update: Add note about encrypted update
firmware_update: Add note about encrypted update

Signed-off-by: Garret Kelly <d5e09c72d3e525ee6ce60f794290ad8eb687c905@google.com>

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