arm: disable frequency invariance for CONFIG_BL_SWITCHER

big.LITTLE switching complicates the setting of a correct cpufreq-based
frequency invariance scale factor due to (as observed in
drivers/cpufreq/vexpress-spc-cpufreq.c):
 - Incorrect current and maximum frequencies as a result of the
   exposure of a virtual frequency table to the cpufreq core,
 - Missed updates as a result of asynchronous frequency adjustments
   caused by frequency changes in other CPU pairs.

Given that its functionality is atypical in regards to frequency
invariance and this is an old technology, disable frequency
invariance for when big.LITTLE switching is configured in to prevent
incorrect scale setting.

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Suggested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>