resource: Introduce resource_assigned() for discerning active resources

A PCI bridge resource lifecycle involves both a "request" and "assign"
phase. At any point in time that resource may not yet be assigned, or may
have failed to assign (because it does not fit).

There are multiple conventions to determine when assignment has not
completed: IORESOURCE_UNSET, IORESOURCE_DISABLED, and checking whether the
resource is parented.

In code paths that are known to not be racing assignment, e.g. post
subsys_initcall(), the most reliable method to judge that a bridge resource
is assigned is to check the resource is parented [1].

Introduce a resource_assigned() helper for this purpose.

Link: http://lore.kernel.org/2b9f7f7b-d6a4-be59-14d4-7b4ffccfe373@linux.intel.com [1]
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20251113021446.436830-4-dan.j.williams@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>