ARM: dts: vf610: enable watchdog for Cortex-A5 dt's

During restructuring of the device tree files the watchdog was
changed to be disabled by default. However, since the watchdog
instance is dedicated to the Cortex-A5, enable the peripheral
by default in the base device tree vf500.dtsi.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>