greth: Move the Aeroflex Gaisler driver

Move the Aeroflex Gaisler driver into drivers/net/ethernet/aeroflex/
and make the necessary Kconfig and Makefile changes.

CC: Kristoffer Glembo <kristoffer@gaisler.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>