docs/arm64: cpu-feature-registers: Documents missing visible fields

A couple of fields visible to userspace are not described in the
documentation. So update it.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>