netdevsim: register couple of devlink params

Register couple of devlink params, one generic, one driver-specific.
Make the values available over debugfs.

Example:
$ echo "111" > /sys/bus/netdevsim/new_device
$ devlink dev param
netdevsim/netdevsim111:
  name max_macs type generic
    values:
      cmode driverinit value 32
  name test1 type driver-specific
    values:
      cmode driverinit value true
$ cat /sys/kernel/debug/netdevsim/netdevsim111/max_macs
32
$ cat /sys/kernel/debug/netdevsim/netdevsim111/test1
Y
$ devlink dev param set netdevsim/netdevsim111 name max_macs cmode driverinit value 16
$ devlink dev param set netdevsim/netdevsim111 name test1 cmode driverinit value false
$ devlink dev reload netdevsim/netdevsim111
$ cat /sys/kernel/debug/netdevsim/netdevsim111/max_macs
16
$ cat /sys/kernel/debug/netdevsim/netdevsim111/test1

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>