net: ethernet: ti: am65-cpsw: add multi port support in mac-only mode

This patch adds final multi-port support to TI AM65x CPSW driver path in
preparation for adding support for multi-port devices, like Main CPSW0 on
K3 J721E SoC or future CPSW3g on K3 AM64x SoC.
- the separate netdev is created for every enabled external Port;
- DMA channels are common/shared for all external Ports and the RX/TX NAPI
and DMA processing assigned to first available netdev;
- external Ports are configured in mac-only mode, which is similar to TI
"dual-mac" mode for legacy TI CPSW - packets are sent to the Host port only
in ingress and directly to the Port on egress. No packet switching between
external ports happens.
- every port supports the same features as current AM65x CPSW on external
device.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>