ARM: dts: imx7s: remove snvs_poweroff node

System poweroff is already supported by PSCI on i.MX7D,
remove "snvs_poweroff" node to avoid below debug message
from syscon-poweroff driver:

[    1.831414] syscon-poweroff 30370000.snvs:snvs-poweroff: pm_power_off already claimed (ptrval) psci_sys_poweroff
[    1.841707] syscon-poweroff: probe of 30370000.snvs:snvs-poweroff failed with error -16

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