arm64: dts: nitrogen8-som: correct i2c1 pad-ctrl

The slew rate and drive-strength of the i2c1 pads were much too
high. Bring them down to avoid signal quality issues.

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