ARM: dts: imx6ul-pico: Describe the Ethernet PHY clock

Since commit c7e73b50 ("ARM: imx: mach-imx6ul: remove 14x14 EVK
specific PHY fixup")thet Ethernet PHY is no longer configured via code
in board file.

This caused Ethernet to stop working.

Fix this problem by describing the clocks and clock-names to the
Ethernet PHY node so that the KSZ8081 chip can be clocked correctly.

Fixes: c7e73b50 ("ARM: imx: mach-imx6ul: remove 14x14 EVK specific PHY fixup")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>