gpu: nova-core: move Firmware to firmware module

We will extend the firmware methods, so move it to its own module
instead to keep gpu.rs focused.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250507-nova-frts-v3-7-fcb02749754d@nvidia.com


[ Don't require a bound device, remove pub visibility from Firmware
  fields, use FIRMWARE_VERSION consistently. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>