[ipgen] Mark the --config-file argument as required
[ipgen] Mark the --config-file argument as required

The script unconditionally reads the configuration file (not
unreasonable: this tells it what to do!), so that argument must be
given.

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

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