BACKPORT: arm64: Extract feature parsing code from cpu_errata.c

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>

Bug: 31432001
Change-Id: I0cee571bf93b5728b5f111ddb1cfe0ebdc3402be
(cherry picked from commit 359b706473b47da3c93bd99fd10d798fe411ab67)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>