raspberry-pi-4: add poe-plus-hat, update poe-hat, use dtmerge instead of fdtoverlay

[carlossless]

Changes here:

• update the poe-hat-overlay dts to work with the 5.15y release of rpis kernel
• add poe-plus-hat-overlay for use with the PoE+ HAT
• using dtmerge from libraspberypi instead of fdtoverlay for the added overlays that require it's the usage

This should provide a ready-to-use solution for:

• Improve device-tree overlay support nixpkgs#79370 (comment)
• https://discourse.nixos.org/t/fdt-err-notfound-when-trying-to-apply-device-tree-overlay-on-raspberry-pi-4/19650

There's likely many ways these expressions could be made better, so feedback is very much appreciated.

[06kellyjac]

Generally good, just some thoughts although the nativeBuildInputs change is strongly recommended :)

[carlossless]

@06kellyjac thanks for the review and all the suggestions, not sure how but my first change batch somehow missed them, but everything should be changed now.