MAINTAINERS: net: Update reviewers for TI's Ethernet drivers

Grygorii is no longer associated with TI and messages addressed to
him bounce.

Add Siddharth, Roger and myself as reviewers.

Signed-off-by: Ravi Gunasekaran <r-gunasekaran@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>