ACPI: glue: Introduce acpi_dev_has_children()

Define acpi_dev_has_children() as a wrapper around
acpi_dev_for_each_child() and use it to check if the given ACPI
device has any children instead of checking the children list
head in struct acpi_device.

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>