arm64: cpufeature: Extend comment to describe absence of field info

When a feature register field is omitted from the description of the
register, the corresponding bits are treated as STRICT RES0, including
for KVM guests. This is subtly different to declaring the field as
HIDDEN/STRICT/EXACT/0, so update the comment to call this out.

Signed-off-by: Will Deacon <will@kernel.org>