MAINTAINERS: net: add entry for TI Ethernet Switch drivers

Add record for TI Ethernet Switch Driver CPSW/CPDMA/MDIO HW
(am33/am43/am57/dr7/davinci) to ensure that related patches
will go through dedicated linux-omap list.

Also add Mugunthan as maintainer and myself as the reviewer.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>