[ci] Run regtool under make -k
[ci] Run regtool under make -k

Now that regtool.py can fail (if something goes wrong when parsing or
generating), it's probably better to keep going for the other blocks
in CI.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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