MAINTAINERS: update for Marvell Ethernet drivers

Marvell has agreed to do maintenance on the sky2 driver.
   * Add the developer to the maintainers file
   * Remove the old reference to the long gone (sk98lin) driver
   * Rearrange to fit current topic organization

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>