ARM: dts: vfxxx: Add SNVS node

Add device tree node for the Secure Non-Volatile Storage
(SNVS) on the VF610 platform. The SNVS block also has a
Real Time Counter (RTC).

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>