tools: ynl: process unknown for enum values

Extend the process_unknown handing to enum values and flags.

Tested by removing entries from rt-link.yaml and rt-neigh.yaml:

./tools/net/ynl/pyynl/cli.py --family rt-link --dump getlink \
    --process-unknown --output-json | jq '.[0] | ."ifi-flags"'
[
  "up",
  "Unknown(6)",
  "loopback",
  "Unknown(16)"
]

./tools/net/ynl/pyynl/cli.py --family rt-neigh --dump getneigh \
    --process-unknown --output-json | jq '.[] | ."ndm-type"'
"unicast"
"Unknown(5)"
"Unknown(5)"
"unicast"
"Unknown(5)"
"unicast"
"broadcast"

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>