arm64: cpufeature: Avoid warnings due to unused symbols

An allnoconfig build complains about unused symbols due to functions
that are called via conditional cpufeature and cpu_errata table entries.

Annotate these as __maybe_unused if they are likely to be generic, or
predicate their compilation on the same option as the table entry if
they are specific to a given alternative.

Signed-off-by: Will Deacon <will.deacon@arm.com>