pch_gbe: Move the OKI Semiconductor driver

Move the OKI Semiconductor driver into driver/net/ethernet/oki-semi/
and make the necessary Kconfig and Makefile changes.

Note: there is no documented maintainer for this driver, so I CC'd
the last 2 major contributors.

CC: Tomoya <tomoya-linux@dsn.okisemi.com>
CC: Toshiharu Okada <toshiharu-linux@dsn.okisemi.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>