ARM: dts: exynos: Add syscon compatible to clock controller on Exynos542x

In order to get the clock by phandle and use it with regmap it needs to be
compatible with syscon. The DMC driver uses two registers from clock
register set and needs the regmap of them.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>