phy: core: Allow children node to be overridden

In order to more flexibly support device tree bindings, allow drivers to
override the container of the child nodes. By default the device node of
the PHY provider is assumed to be the parent for children, but bindings
may decide to add additional levels for better organization.

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>