ARM: dts: imx7s-warp: Fix watchdog pinmux

The watchdog pin is controlled by the iomuxc_lpsr, not the regular
iomux, so move it there.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>