[vendor.py] Enable overriding of upstream.rev
[vendor.py] Enable overriding of upstream.rev

A typo in the allow list prevented overriding of the upstream.rev key.

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

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