Add device node of SROM controller for exynos4.

CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: Ian Campbell <ijc+devicetree@hellion.org.uk>
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>
[k.kozlowski: fixed size of mapped SROMC memory region]
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>