driver: net: ethernet: cpsw: dual emac interface implementation

The CPSW switch can act as Dual EMAC by segregating the switch ports
using VLAN and port VLAN as per the TRM description in
14.3.2.10.2 Dual Mac Mode

Following CPSW components will be common for both the interfaces.
* Interrupt source is common for both eth interfaces
* Interrupt pacing is common for both interfaces
* Hardware statistics is common for all the ports
* CPDMA is common for both eth interface
* CPTS is common for both the interface and it should not be enabled on
  both the interface as timestamping information doesn't contain port
  information.

Constrains
* Reserved VID of One port should not be used in other interface which will
  enable switching functionality
* Same VID must not be used in both the interface which will enable switching
  functionality

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>