cpufreq: interactive: Fix handling of SMP where CPUs must scale in tandem

The interactive governor relies on quirks of the Tegra 2 cpufreq
implementation for handling SMP systems where the CPUs do not have
separate rate controls.  It needs to determine the maximum rate
for all CPUs covered by the policy and set that speed.

Change-Id: I1ed9fa171e5a9c45a1fa5944e3fa823eb157e81f
Signed-off-by: Todd Poynor <toddpoynor@google.com>