tools: ynl: add a lint makefile target

tools: ynl: add a lint makefile target

Add a lint target to run yamllint on the YNL specs.

make -C tools/net/ynl lint
make: Entering directory '/home/donaldh/net-next/tools/net/ynl'
yamllint ../../../Documentation/netlink/specs/*.yaml
../../../Documentation/netlink/specs/ethtool.yaml
  1272:21   warning  truthy value should be one of [false, true]  (truthy)

make: Leaving directory '/home/donaldh/net-next/tools/net/ynl'

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20251127123502.89142-3-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>