ARM: dts: imx6qdl-pico: Fix Ethernet support

Currently, it is no longer possible to retrieve a DHCP address
on the imx6qdl-pico board.

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: 98670a0b ("ARM: dts: imx6qdl: Add imx6qdl-pico support")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>