ARM: dts: vf610twr: assign Ethernet clock explicitly

Assign Ethernet clock parents explicitly. The VF610 Tower Board
uses the external Ethernet clock input which is connected to
a 50MHz clock.

The Vybrid SoC has two ethernet interfaces (fec0 and fec1) which
use the same clock source (VF610_CLK_ENET). Therefore this parent
configuration affects multiple consumer devices and need to be
specified in the clock provider node.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>