boards: hifive_unmatched: add support for S7 and U74 targets

Add `hifive_unmatched//s7` (earlier selected by default, using
`hifive_unmatched`) and `hifive_unmatched//u74` targets.

Define work-area for other 4 cores in openocd.cfg

Update twister platform white/black lists, to support new targets

Signed-off-by: Jakub Wasilewski <jwasilewski@internships.antmicro.com>
Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>