mlxsw: core: Add devlink shared buffer callbacks

Add middle layer in mlxsw core code to forward shared buffer calls
into specific ASIC drivers.

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