pcengines-apu-firmware: Update to version 4.15.0.1

pcengines-apu-firmware: Update to version 4.15.0.1

- Update from 4.14.0.4 to 4.15.0.1
- Update of rootfile
- Changelog
    v4.15.0.1
        rebased with official coreboot repository commit 6973a3e7
    v4.14.0.6
        rebased with official coreboot repository commit d06c0917
        Re-added GPIO bindings to fix LED and button functionality
    v4.14.0.5
        rebased with official coreboot repository commit d4c55353
        Updated CPU declarations in ACPI to comply with newer ACPI standard
        Removed GPIO bindings to fix conflict with OS drivers

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>