ARM: dts: vf-colibri: assign Ethernet clock explicitly

Assign Ethernet clock parents explicitly. The Colibri VF61
uses the 50MHz Ethernet clock provided by PLL5.

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>