device property: Split fwnode_get_child_node_count()

The new helper is introduced to allow counting the child firmware nodes
of their parent without requiring a device to be passed. This also makes
the fwnode and device property API more symmetrical with the rest.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20250310150835.3139322-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Lee Jones <lee@kernel.org>