MAINTAINERS: add myself as Renesas Ethernet drivers reviewer

Add myself as  a reviewer for the Renesas Ethernet drivers -- hopefully I
won't miss the buggy  patches anymore. :-)

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>