ARM: dts: imx7d-zii-rpu2: Remove unneeded snvs_pwrkey node

Since commit 4664179f ("ARM: dts: imx7s: Enable SNVS power key
according to board design") snvs_pwrkey is disabled by default, so there
is no need for disabling it explicitly in the board dts anymore.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>