boards: hifive_unleashed: add support for E51 and U54 targets

Add `hifive_unleashed//e51` (earlier selected by default, using
`hifive_unleashed`) and `hifive_unleashed//u54` 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>