clk: mvebu: armada-37xx-periph: add DVFS support for cpu clocks

When DVFS is enabled the CPU clock setting is done using an other set of
registers.

These Power Management registers are exposed through a syscon as they
will also be used by other drivers such as the cpufreq.

This patch add the possibility to modify the CPU frequency using the
associate load level matching the target frequency. Then all the
frequency switch is handle by the hardware.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
[sboyd@codeaurora.org: Grow a local variable for regmap pointer
to keep lines shorter]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>