NET: add ENC28J60 driver using SPI framework

V3: further refinements:
- use priv member instead of container method
- allow setting of MAC address by write_hwaddr method
- avoid shutting down link between commands

Signed-off-by: Reinhard Meyer <u-boot@emk-elektronik.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>