devicetree: bindings: update DT bindings for Marvell EBU clock support

With the introduction of the Marvell Armada 39x SoC, the DT bindings
for Marvell EBU clocks need to be extended. This commit include the
corresponding update to the Device Tree bindings documentation.

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