clk: mvebu: extend clk-cpu for dynamic frequency scaling

This commit extends the existing clk-cpu driver used on Marvell Armada
XP platforms to support the dynamic frequency scaling of the CPU
clock. Non-dynamic frequency change was already supported (and used
before secondary CPUs are started), but the dynamic frequency change
requires a completely different procedure.

In order to achieve this, the clk_cpu_set_rate() function is reworked
to handle two separate cases:

 - The case where the clock is enabled, which is the new dynamic
   frequency change code, implemented in clk_cpu_on_set_rate(). This
   part will be used for cpufreq activities.

 - The case where the clock is disabled, which is the existing
   frequency change code, moved in clk_cpu_off_set_rate(). This part
   is already used to set the clock frequency of the secondary CPUs
   before starting them.

In order to implement the dynamic frequency change function, we need
to access the PMU DFS registers, which are outside the currently
mappe...