PM: EM: Don't search matching freq. in em_update_performance_limits()

The CPPC cpufreq drivers relies on a abstract continuous scale of
performance levels. All performance level can be requested between
the min/max values.
The CPPC cpufreq drivers can also create an artificial Energy Model
(EM). OPP frequencies being artificial, they don't necessarily have
a matching hardware frequency.

Relax the condition in em_update_performance_limits() searching
for an exact match. Use the closest frequency satisfying the min/max
request instead.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>