pm: state: allow disabling certain power states

In some platforms it may be desirable to disable certain CPU power
states, for example, because they have extra requirements not available
on all boards/applications. Because `cpu-power-states` are defined at
SoC dts file levels, the only way to achieve that now was by re-defining
`cpu-power-states` property in e.g. a board file. With this patch, one
can now selectively set `status = "disabled";` to any power state and it
will be skipped by the PM subsystem.

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