net: broadcom: bcm4908enet: add BCM4908 controller driver

BCM4908 SoCs family uses Ethernel controller that includes UniMAC but
uses different DMA engine (than other controllers) and requires
different programming.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>