clk: mvebu: armada-37xx-periph: prepare cpu clk to be used with DVFS

When DVFS will be enabled then the cpu clk will use a different set of
register at run time. That means that we won't be able to use the common
callback and need to use our own ones.

This patch prepares this change by switching on our own set of callbacks
without modifying the behavior of the clocks.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>