Currently, in an overlay fdt fragment, it needs to specify the exact
location in base DT. In another word, when the fdt fragment is generated,
the base DT location for the fragment is already known.

There is new use case that the base DT location is unknown when fdt
fragment is generated. For example, the add-on device provide a fdt
overlay with its firmware to describe its downstream devices. Because it
is add-on device which can be plugged to different systems, its firmware
will not be able to know the overlay location in base DT. Instead, the
device driver will load the overlay fdt and apply it to base DT at runtime.
In this case, of_overlay_fdt_apply() needs to be extended to specify
the target node for device driver to apply overlay fdt.
    int overlay_fdt_apply(..., struct device_node *base);
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://lore.kernel.org/r/1692120000-46900-5-git-send-email-lizhi.hou@amd.comSigned-off-by: Rob Herring <robh@kernel.org>