tehuti: Move the Tehuti driver

Move the Tehuti driver into drivers/net/ethernet/tehuti/ and
make the necessary Kconfig and Makefile changes.

CC: Alexander Indenbaum <baum@tehutinetworks.net>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>