MAINTAINERS: Update APM X-Gene section

Updated APM X-Gene ethernet driver maintainers list.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>