dpll: spec: use proper enum for pin capabilities attribute

The enum is defined, however the pin capabilities attribute does
refer to it. Add this missing enum field.

This fixes ynl cli output:

Example current output:
$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/dpll.yaml --do pin-get --json '{"id": 0}'
{'capabilities': 4,
 ...
Example new output:
$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/dpll.yaml --do pin-get --json '{"id": 0}'
{'capabilities': {'state-can-change'},
 ...

Fixes: 3badff3a

 ("dpll: spec: Add Netlink spec in YAML")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20240306120739.1447621-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>