[ipgen] Explicitly load config files as UTF-8
[ipgen] Explicitly load config files as UTF-8

We explicitly save configuration files as UTF-8, ensure we load them the
same way.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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