cpufreq: interactive governor drops bits in time calculation

Keep time calculation in 64-bit throughout. If we have long times
between idle calculations this can result in deltas > 32 bits
which causes incorrect load percentage calculations and selecting
the wrong frequencies if we truncate here.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Alex Naidis <alex.naidis@linux.com>