gpu: nova-core: allow register aliases

gpu: nova-core: allow register aliases

Some registers (notably scratch registers) don't have a definitive
purpose, but need to be interpreted differently depending on context.

Expand the register!() macro to support a syntax indicating that a
register type should be at the same offset as another one, but under a
different name, and with different fields and documentation.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250619-nova-frts-v6-9-ecf41ef99252@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>