linuxkpi: Add to_acpi_device_node() and ACPI_COMPANION()

The former is called by the latter. We return NULL because linuxkpi does
not implement ACPI (pseudo?) devices associated to regular devices.

The amdgpu DRM driver started to use ACPI_COMPANION() in Linux 6.13.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57577