arm64: spectre: Consolidate spectre-v3a detection

The spectre-v3a mitigation is split between cpu_errata.c and spectre.c,
with the former handling detection of the problem and the latter handling
enabling of the workaround.

Move the detection logic alongside the enabling logic, like we do for the
other spectre mitigations.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20201113113847.21619-10-will@kernel.org