clk: mvebu: add Marvell Armada 39x driver

This commit adds a new clock driver for the Marvell Armada 39x family
of processors. This driver is fairly similar to the ones already used
on other Marvell EBU processors, with the following main differences:

 * Different set of ratios
 * Different set of core clocks
 * Configurable reference clock in frequency

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