ACPI: bus: Export acpi_dev_for_each_child() to modules

Some pieces of modular code can benefit from using
acpi_dev_for_each_child(), so export it to modules.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>