ARM: dts: lpc4350-hitex-eval: add ethernet

Enable Ethernet and add pin muxing and set the correct
frequency on the enet tx clock input.

Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>