cpufreq: db8500: set CPUFREQ_CONST_LOOPS

As ux500 is being converted to timer based delay loops, and the timer
used is not depending on CPUs clock frequency, set cpufreq_driver flag
CPUFREQ_CONST_LOOPS to prevent cpufreq rescaling loops_for_jiffies.

Acked-by: John Stultz <john.stultz@linaro.org>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>