arm64: dts: marvell: update Armada AP806 clock description

Following the review from the DT maintainers, the DT binding for the
clocks has changed, and we now use a DFX server node exposing a
syscon, with the clock nodes being subnodes of the DFX server
node. This commit therefore updates the AP806 Device Tree file to use
this new DT binding.

[gregory.clement@free-electrons.com: Fix commit title by adding ' dts:']
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>