[NET]: Add support for the RDC R6040 Fast Ethernet controller

This patch adds support for the RDC R6040 MAC we can find in the RDC
R-321x System-on-chips.

Signed-off-by: Sten Wang <sten.wang@rdc.com.tw>
Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>