ARM: dts: exynos: Prepare for inclusion of exynos5.dtsi in exynos5410.dtsi

Add 'sromc' label to SROM memory controller in common exynos5.dtsi so it
can be referenced by inheriting DTSI.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>