tools: ynl: allow user to pass enum string instead of scalar value

During decoding of messages coming from kernel, attribute values are
converted to enum names in case the attribute type is enum of bitfield32.

However, when user constructs json message, he has to pass plain scalar
values. See "state" "selector" and "value" attributes in following
examples:

$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/dpll.yaml --do pin-set --json '{"id": 0, "parent-device": {"parent-id": 0, "state": 1}}'
$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/devlink.yaml --do port-set --json '{"bus-name": "pci", "dev-name": "0000:08:00.1", "port-index": 98304, "port-function": {"caps": {"selector": 1, "value": 1 }}}'

Allow user to pass strings containing enum names, convert them to scalar
values to be encoded into Netlink message:

$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/dpll.yaml --do pin-set --json '{"id": 0, "parent-device": {"parent-id": 0, "state": "connected"}}'
$ sudo ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/devlink.yaml --do port-set --json '{"bus-name": "pci", "dev-name": "0000:08:00.1", "port-index": 98304, "port-function": {"caps": {"selector": ["roce-bit"], "value": ["roce-bit"] }}}'

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240222134351.224704-4-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>