dpll: extend uapi by lock status error attribute

If the dpll devices goes to state "unlocked" or "holdover", it may be
caused by an error. In that case, allow user to see what the error was.
Introduce a new attribute and values it can carry.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>