arm64/cpufeature: Pull out helper for CPUID register definitions

We use the same structure to match hwcaps and CPU features so we can use
the same helper to generate the fields required. Pull the portion of the
current hwcaps helper that initialises the fields out into a separate
define placed earlier in the file so we can use it for cpufeatures.

No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230303-arm64-cpufeature-helpers-v2-1-4c8f28a6f203@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>