ARM: dts: imx6qdl-wandboard: Fix Ethernet support

Currently, the imx6q-wandboard Ethernet does not transmit any
data.

This issue has been exposed by commit f5d9aa79 ("ARM: imx6q:
remove clk-out fixup for the Atheros AR8031 and AR8035 PHYs").

Fix it by describing the qca,clk-out-frequency property as suggested
by the commit above.

Fixes: 77591e42

 ("ARM: dts: imx6qdl-wandboard: add ethernet PHY description")
Signed-off-by: Martin Haaß <vvvrrooomm@gmail.com>
Tested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>