KVM: arm64: Advertise selected DebugVer in DBGDIDR.Version

Much like we do for other fields, extract the Debug architecture version
from the ID register to populate the corresponding field in DBGDIDR.
Rewrite the existing sysreg field extractors to use SYS_FIELD_GET() for
consistency.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>