arm64: dts: nitrogen8-som: correct network PHY reset

Add the missing reset-gpios property to allow Linux to fully reset
the network PHY and fix the pinmux to add the neccessary pull-ups
for the PHY strap configuration.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>