clk: mvebu: add CPU clock driver for Armada 7K/8K

The CPU frequency is managed at the AP level for the Armada 7K/8K. The
CPU frequency is modified by cluster: the CPUs of the same cluster have
the same frequency.

This patch adds the clock driver that will be used by CPUFreq, it is
based on the work of Omri Itach <omrii@marvell.com>.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lkml.kernel.org/r/20190710134346.30239-4-gregory.clement@bootlin.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>