[reggen] Prefer explicit utf-8 encoding
[reggen] Prefer explicit utf-8 encoding

This fixes the chip level nightly breakage.

Signed-off-by: Srikrishna Iyer <2803d640feace36379942447842f26c7747b4dc3@google.com>

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