ACPI: scan: Walk ACPI device's children using driver core

Instead of walking the list of children of an ACPI device directly, use
acpi_dev_for_each_child() or acpi_dev_for_each_child_reverse() to carry
out an action for all of the given ACPI device's children.

This will help to eliminate the children list head from struct
acpi_device as it is redundant and it is used in questionable ways
in some places (in particular, locking is needed for walking the
list pointed to it safely, but it is often missing).

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