net: mv643xx_eth: Simplify with scoped for each OF child loop

Use scoped for_each_available_child_of_node_scoped() when iterating
over device nodes to make code a bit simpler.

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>