Require pyyaml >= 5.1
Require pyyaml >= 5.1

This is needed by primgen.py, which uses the sort_keys argument to
dump_all (which was only added in this release).

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

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