Update FENCE/FENCE.I decoding

[mkurc-ant]

This PR updates the instruction decoder so that it does not treat FENCE / FENCE.I with non-zero supplementary arguments as illegal. This behavior conforms to the spec in version "20190608-Base-Ratified" which VeeR is compatible with.

With that PR I also updated the instructions on how to re-generate instruction decoder equations.

Solves https://github.com/chipsalliance/Cores-VeeR-EL2/issues/128

[github-actions[bot]]

Links to coverage and verification reports for this PR (#138) are available at https://chipsalliance.github.io/Cores-VeeR-EL2/

[tmichalak]

LGTM