pm: device: helper for running actions on children

Adds a helper function for running PM actions on child devices. A custom
implementation of `device_supported_foreach` is used as we don't
necessarily want the early exit behaviour of that function in the power
domain context.

The early exit behaviour can be obained through a return value in the
failure callback if that is desired.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>