arm64: dts: imx8qxp: Move watchdog node into scu node

i.MX system controller watchdog has pretimeout function which
depends on i.MX SCU driver, so it should be a subnode of SCU.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>