ARM: dts: vf610-zii: Disable SNVS RTC

None of these vf610-zii boards have a battery or super-capacitor
holding up power to the SNVS RTC embedded in the Vybrid SoC,
so it is preferable to disable the snvsrtc node.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>