ARM: dts: update watchdog node name in exynos5440

Made it as per DT node naming convention <name@reg_addr>.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>