arm64: dts: imx8mm-verdin: correct off-on-delay

The property should be off-on-delay-us, not off-on-delay

Fixes: 6a57f224 ("arm64: dts: freescale: add initial support for verdin imx8m mini")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>