ARM: dts: mvebu: Move mv98dx3236 clock bindings

Previously the coreclk binding for the 98dx3236 SoC was inherited from
the armada-370/xp. This block is present in as much as it is possible to
read from the register location without causing any harm. However the
actual sampled at reset values are reflected in the DFX block.

Moving the binding to the DFX block enables support for different clock
strapping options in hardware.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>