devlink: move code to a dedicated directory

devlink: move code to a dedicated directory

The devlink code is hard to navigate with 13kLoC in one file.
I really like the way Michal split the ethtool into per-command
files and core. It'd probably be too much to split it all up,
but we can at least separate the core parts out of the per-cmd
implementations and put it in a directory so that new commands
can be separate files.

Move the code, subsequent commit will do a partial split.

Reviewed-by: Jacob Keller <[email protected]>
Reviewed-by: Jiri Pirko <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>