ARM: dts: Add syscon property to the MIPI phy in exynos5420

Since commit e4b3d380 ("phy: exynos-video-mipi: Fix regression by
adding support for PMU regmap") the syscon property is required in
samsung,s5pv210-mipi-video-phy nodes, but this DTS hadn't been updated
yet.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>