arm64: dts: imx8mp-phycore-som: Update WDOG muxing

To be able to trigger a reset also from an external source we
need to configure the WDOG pin as open drain.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>