ARM: dts: exynos: Add status property to Exynos 542x Mixer nodes

HDMI support requires some additional off-SoC logic, so Mixer device (part
of HDMI display path) should be disabled by default in SoC dtsi and enabled
then in each board dts. This patch unifies Mixer handling with other
Exynos SoCs.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>