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.

Change-Id: I8ac9705b9f166a95715c985efacd2ae8ef387cb7
Signed-off-by: Narayanan Gopalakrishnan <nargop@codeaurora.org>