pm: s/power-domain/power-domains and add power-domain-names

Some devices may belong to >1 power domain, so with the current design
this is something not possible to describe. It's worth to note that
Linux also uses the `power-domains` naming scheme, not `power-domain`.
This patch also introduces `power-domain-names` so that each entry in
`power-domains` can be given a name if needed. `#power-domain-cells`
is now required as well.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>