driver core: Move two simple APIs for finding child device to header

driver core: Move two simple APIs for finding child device to header

The following two APIs are for finding child device, and both only have
one line code in function body.
device_find_child_by_name()
device_find_any_child()

Move them to header as static inline function.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/r/20250105-class_fix-v6-8-3a2f1768d4d4@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>