[topgen] Return a list as annotated
[topgen] Return a list as annotated

The `.keys()` method returns a dict_keys object, not a list, as
annotated in the function header. Make the function actually return a
list.

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

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