overlay: Resolve phandle fixups in child nodes

[arthokal]

Issue:
When user overlay files (passed via -i) contain child nodes with phandle references, those references are silently dropped in the generated pl.dtsi

For example, updated_zocl.dtsi contains:

&amba_pl {                                                                      
        zyxclmm_drm {
                compatible = "xlnx,zocl-versal";
                xlnx,memory-region = <&cma_reserved>,<&ddr_ch1>,<&ddr_ch2>,<&ddr_ch3>;
        };
};

The generated overlay has zyxclmm_drm with xlnx,memory-region missing.

Root Cause:
_unwrap_overlay_tree() in lopper/__init__.py resolves 0xffffffff phandle placeholders (left by dtc plugin compilation) only for direct properties of the __overlay__ node.
Child node properties (like those in zyxclmm_drm) are deep-copied as-is with unresolved 0xffffffff values, which are silently dropped at write time.

Fix:
Instead of iterating properties and searching for matching fixups (inside-out), iterate label_to_fixups and patch nodes directly via ov_tree[path] (outside-in). This resolves phandles across overlay` and all its children in one pass before any copying happens.

Example Lopper command:

LOPPER_DTC_FLAGS="-b 0 -@" ./lopper.py -O outdir -f --enhanced -i /proj/ssw_xhd/drivers/onkarh/to_aravind/updated_zocl.dtsi /proj/ssw_xhd/drivers/onkarh/to_aravind/apu_linux-chosen.dts system-top-no-pl.dts -- xlnx_overlay_pl_dt psv_cortexa72_0 full --firmware-name=gmio_async_xrt.pdi

[arthokal]

@onkarharsh,
Please review.

[zeddii]

The current resolution architecture was on purpose and should only be done on write time.

I don't have access to my full setup to test this, as I'm on vacation this week.

Does this change preserve the phandle resolution at write time ? I can't tell from looking at the patch.

Could you also attach some sample inputs and expected outputs ? So I can reproduce and test the issue myself.