mfd: db8500-prcmu: Get rid of cpufreq dependency

The ARMSS clock, also known as the operating point of the
CPU, should not cross-depend on cpufreq like this. Move
the code to use just frequencies and remove the false
frequency (1GHz) and put in the actual frequency provided
by the ARMSS clock (998400000 Hz) as part of the process.

After this and the related cpufreq patch, the DB8500 will
simply use the standard DT cpufreq driver to change the
operating points through the common clock framework using
the ARMSS clock.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>