ARM: dts: armada-xp-rn2120: add pinmuxing for ethernet

Up to now working ethernet depended on the bootloader to configure the
pinmuxing. Make it explicit.

As a side effect this change makes ethernet work in barebox.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>