arm64: Add BBM Level 2 cpu feature

arm64: Add BBM Level 2 cpu feature

The Break-Before-Make cpu feature supports multiple levels (levels 0-2),
and this commit adds a dedicated BBML2 cpufeature to test against
support for.

To support BBML2 in as wide a range of contexts as we can, we want not
only the architectural guarantees that BBML2 makes, but additionally
want BBML2 to not create TLB conflict aborts. Not causing aborts avoids
us having to prove that no recursive faults can be induced in any path
that uses BBML2, allowing its use for arbitrary kernel mappings.

This feature builds on the previous ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE,
as all early cpus must support BBML2 for us to enable it (and any later
cpus must also support it to be onlined).

Not onlining late cpus that do not support BBML2 is unavoidable, as we
might currently be using BBML2 semantics for kernel memory regions. This
could cause faults in the late cpus, and would be difficult to unwind,
so let us avoid the case altogether.

Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20250625113435.26849-3-miko.lenczewski@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>