linuxkpi: Add dev_is_removable()

[Why]
This is used by the amdgpy DRM driver starting from Linux 6.7.

[How]
The function always returns false, like pci_is_thunderbolt_attached()
because we don't have an API for this in FreeBSD yet.

Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48758