NET: arc-emac: support for a supply regulator (needed on Radxa Rock)

Support a 'phy-supply' property in devicetree. This can be used to defer
probing until the regulator is available and could also be used later
for runtime power management.
dts: import DTS for Radxa Rock from Heiko Stübner
fix clock placement
import Heiko Stübner's clock initialization code

This makes the EMAC work again - it apparently got an unsupported clock
frequency before.