net: ti: am65-cpsw: Add support for multi port independent MAC mode

On certain TI SoC, like AM64x there is a CPSW3G which supports 2
external independent MAC ports for single CPSW instance.
It is not possible for Ethernet driver to register more than one port
for given instance.

This patch modifies top level CPSW NUSS as UCLASS_MISC and binds
UCLASS_ETH to individual ports so as to support bring up more than one
Ethernet interface in U-Boot.

Note that there is no isolation in the since, CPSW NUSS is in promisc
mode and forwards all packets to host.

Since top level driver is now UCLASS_MISC, board files would need to
instantiate this driver explicitly.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>