ARM: dts: Remove obsolete MIPI DPHY 'reg' property for exynos4

Since e4b3d380 ("phy: exynos-video-mipi: Fix regression by
adding support for PMU regmap") the Exynos PMU driver provides
regmap to access the MIPI DPHY registers. The MIPI DPHY node uses
a phandle to syscon to get this regmap. The 'reg' field is obsolete.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>