Add support for Raspberry Pi 4 in device tree mode

[markafarrell]

Add support for Raspberry Pi 4 in device tree mode

Add support for running flatcar on Raspberry Pi 4 in device tree mode

Fixes 1750

How to use

1. Follow instructions as per https://www.flatcar.org/docs/latest/installing/bare-metal/raspberry-pi/
2. Enable device tree mode in UEFI

virt-fw-vars \
    --input RPI_EFI.fd \
    --output RPI_EFI.fd \
    --set-json <(cat <<EOF
{
    "version": 2,
    "variables": [
        {
            "name": "RamLimitTo3GB",
            "guid": "cd7cc258-31db-22e6-9f22-63b0b8eed6b5",
            "data": "00000000"
        },
        {
            "name": "SystemTableMode",
            "guid": "cd7cc258-31db-22e6-9f22-63b0b8eed6b5",
            "data": "01000000"
        }
    ]
}
EOF
)

3. Add pcie-32bit-dma overlay

curl -L \
    https://github.com/raspberrypi/firmware/raw/refs/heads/master/boot/overlays/pcie-32bit-dma.dtbo \
    --output /tmp/efipartition/overlays/pcie-32bit-dma.dtbo
echo "dtoverlay=pcie-32bit-dma" >> config.txt

4. Add rpi-poe overlay

curl -L \
    https://github.com/raspberrypi/firmware/raw/refs/heads/master/boot/overlays/rpi-poe.dtbo \
    --output /tmp/efipartition/overlays/rpi-poe.dtbo
echo "dtoverlay=rpi-poe >> config.txt

5. Set acpi=off kernel parameter in butane configuration

kernel_arguments:
  should_exist:
<snip>
    - acpi=off

Testing done

[Describe the testing you have done before submitting this PR. Please include both the commands you issued as well as the output you got.]

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.