arm64/errata: add REVIDR handling to framework

In some cases, core variants that are affected by a certain erratum
also exist in versions that have the erratum fixed, and this fact is
recorded in a dedicated bit in system register REVIDR_EL1.

Since the architecture does not require that a certain bit retains
its meaning across different variants of the same model, each such
REVIDR bit is tightly coupled to a certain revision/variant value,
and so we need a list of revidr_mask/midr pairs to carry this
information.

So add the struct member and the associated macros and handling to
allow REVIDR fixes to be taken into account.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>