arm64: errata: Don't enable workarounds for "rare" errata by default

arm64: errata: Don't enable workarounds for "rare" errata by default

Arm classifies some of its CPU errata as "rare", indicating that the
hardware error is unlikely to occur in practice. Given that the cost of
errata workarounds can often be significant in terms of power and
performance, don't enable workarounds for "rare" errata by default and
update our documentation to reflect that.

Cc: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240209183916.25860-1-will@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>