gpu: nova-core: register: redesign relative registers

The relative registers are currently very unsafe to use: callers can
specify any constant as the base address for access, meaning they can
effectively interpret any I/O address as any relative register.

Ideally, valid base addresses for a family of registers should be
explicitly defined in the code, and could only be used with the relevant
registers

This patch changes the relative register declaration from e.g.:

    register!(CPU_CTL @ +0x0000010, "CPU core control" {
        0:0     start as bool, "Start the CPU core";
    });

into:

    register!(CPU_CTL @ CpuCtlBase[0x10], "CPU core control" {
        0:0     start as bool, "Start the CPU core";
    });

Where `CpuCtlBase` is the name of a ZST used as a parameter of the
`RegisterBase<>` trait to define a trait unique to a class of register.
This specialized trait is then implemented for every type that provides
a valid base address, enabling said types to be passed as the base
address provider for the register's I/O accessor methods.

This design thus makes it impossible to pass an unexpected base address
to a relative register, and, since the valid bases are all known at
compile-time, also guarantees that all I/O accesses are done within the
valid bounds of the I/O range.

[acourbot@nvidia.com: add example in the commit log.]

Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250718-nova-regs-v2-15-7b6a762aa1cd@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

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