net: mellanox: add DEVLINK dependencies

The new NET_DEVLINK infrastructure can be a loadable module, but the drivers
using it might be built-in, which causes link errors like:

drivers/net/built-in.o: In function `mlx4_load_one':
:(.text+0x2fbfda): undefined reference to `devlink_port_register'
:(.text+0x2fc084): undefined reference to `devlink_port_unregister'
drivers/net/built-in.o: In function `mlxsw_sx_port_remove':
:(.text+0x33a03a): undefined reference to `devlink_port_type_clear'
:(.text+0x33a04e): undefined reference to `devlink_port_unregister'

There are multiple ways to avoid this:

a) add 'depends on NET_DEVLINK || !NET_DEVLINK' dependencies
   for each user
b) use 'select NET_DEVLINK' from each driver that uses it
   and hide the symbol in Kconfig.
c) make NET_DEVLINK a 'bool' option so we don't have to
   list it as a dependency, and rely on the APIs to be
   stubbed out when it is disabled
d) use IS_REACHABLE() rather than IS_ENABLED() to check for
   NET_DEVLINK in include/net/devlink.h

This implements a variation of approach a) by adding an
intermediate symbol that drivers can depend on, and changes
the three drivers using it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 09d4d087 ("mlx4: Implement devlink interface")
Fixes: c4745500 ("mlxsw: Implement devlink interface")
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>