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.

Change-Id: Ia9e0b8f14a1472001a922f7accb53e6a0da4d0a0
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
(cherry picked from commit 6696986a932a28b17858279c5eb69f83d566d4de)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>