arm: dts: Use pmu_system_controller phandle for dp phy

DP PHY now require pmu-system-controller to handle PMU register
to control PHY's power isolation. Adding the same to dp-phy
node.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>