nova: add initial driver stub

Add the initial driver stub of Nova, a Rust-based GSP-only driver for
Nvidia GPUs. Nova, in the long term, is intended to serve as the
successor of Nouveau for GSP-firmware-based GPUs. [1]

As a stub driver Nova's focus is to make use of the most basic device /
driver infrastructure required to build a DRM driver on the PCI bus and
serve as demonstration example and justification for this
infrastructure.

In further consequence, the idea is to develop Nova continuously
upstream, using those increments to lift further Rust abstractions and
infrastructure upstream.

Link: https://lore.kernel.org/dri-devel/Zfsj0_tb-0-tNrJy@cassiopeiae/T/#u

 [1]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>