gpu: nova-core: rename Imem to ImemSecure

gpu: nova-core: rename Imem to ImemSecure

Rename FalconMem::Imem to ImemSecure to indicate that it references
Secure Instruction Memory.  This change has no functional impact.

On Falcon cores, pages in instruction memory can be tagged as Secure
or Non-Secure.  For GA102 and later, only Secure is used, which is why
FalconMem::Imem seems appropriate.  However, Turing firmware images
can also contain non-secure sections, and so FalconMem needs to support
that.  By renaming Imem to ImemSec now, future patches for Turing support
will be simpler.

Nouveau uses the term "IMEM" to refer both to the Instruction Memory
block on Falcon cores as well as to the images of secure firmware
uploaded to part of IMEM.  OpenRM uses the terms "ImemSec" and "ImemNs"
instead, and uses "IMEM" just to refer to the physical memory device.

Renaming these terms allows us to align with OpenRM, avoid confusion
between IMEM and ImemSec, and makes future patches simpler.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260122222848.2555890-2-ttabi@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>