This change has been informed by advice from the lowRISC legal
committee.

The Solderpad 0.51 license states "the Licensor permits any Work
licensed under this License, at the option of the Licensee, to be
treated as licensed under the Apache License Version 2.0". We use this
freedom to convert license markings to Apache 2.0. This commit ensures
that we retain all authorship and copyright attribution information.