cpufreq: fix interactive bug during hotplug.

When a new cpu is added and if does not enter idle
for long, the interactive timer is not started and hence
the cpu remains at lowest frequecny, despite the system
load. This impacts mainly benchmarks. Starting a timer
at governor start to handle low frequency.

Signed-off-by: Narayanan Gopalakrishnan <nargop@codeaurora.org>
(cherry picked from commit 940d4b98eab2577d6c1e710f2939a2b0b6b3f028)