net: lan966x: Add offload support for cbs

Lan966x switch supports credit based shaper in hardware according to
IEEE Std 802.1Q-2018 Section 8.6.8.2. Add support for cbs configuration
on egress port of lan966x switch.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>