arm64: dts: marvell: use new binding for the system controller on ap806

The new binding for the system controller on ap806 moved the clock into a
subnode. This preliminary step will allow to add gpio and pinctrl
subnodes

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>