arc_emac: add clock handling

This adds ability for the arc_emac to really handle its supplying clock.
To get the needed clock-frequency either a real clock or the previous
clock-frequency property must be provided.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Max Schwarz <max.schwarz@online.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc Documentation/devicetree/bindings/net/arc_emac.txt
Simple merge

diff --cc drivers/net/ethernet/arc/emac.h
Simple merge

diff --cc drivers/net/ethernet/arc/emac_main.c
Simple merge