devlink: Add 'fw_load_policy' generic parameter

Many drivers load the device's firmware image during the initialization
flow either from the flash or from the disk. Currently this option is not
controlled by the user and the driver decides from where to load the
firmware image.

'fw_load_policy' gives the ability to control this option which allows the
user to choose between different loading policies supported by the driver.

This parameter can be useful while testing and/or debugging the device. For
example, testing a firmware bug fix.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>