devlink: Add support for get/set driverinit value

"driverinit" configuration mode value is held by devlink to enable
the driver query the value after reload. Two additional functions
added to help the driver get/set the value from/to devlink:
devlink_param_driverinit_value_set() and
devlink_param_driverinit_value_get().

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