net: stmmac: dwmac-sun8i: Use for_each_child_of_node_scoped()

Avoid need to manually handle of_node_put() by using
for_each_child_of_node_scoped(), which can simplfy code.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>