arm64: Extract feature parsing code from cpu_errata.c

commit 359b706473b47da3c93bd99fd10d798fe411ab67 upstream.

As we detect more architectural features at runtime, it makes
sense to reuse the existing framework whilst avoiding to call
a feature an erratum...

This patch extract the core capability parsing, moves it into
a new file (cpufeature.c), and let the CPU errata detection code
use it.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
[david: trivial merge fixups]
Signed-off-by: David Brown <david.brown@linaro.org>