ARM: dts: Fix sysreg node name in exynos4.dtsi

Fix the name as per DT node naming convention.
- rename the node to syscon which is a more generic name.
- append the register value to the node name.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>