cpufreq: tegra194: remove opp table in exit hook

Add exit hook and remove OPP table when the device gets unregistered.
This will fix the error messages when the CPU FREQ driver module is
removed and then re-inserted. It also fixes these messages while
onlining the first CPU from a policy whose all CPU's were previously
offlined.

 debugfs: File 'cpu5' in directory 'opp' already present!
 debugfs: File 'cpu6' in directory 'opp' already present!
 debugfs: File 'cpu7' in directory 'opp' already present!

Fixes: f41e1442 ("cpufreq: tegra194: add OPP support and set bandwidth")
Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
[ Viresh: Dropped irrelevant change from it ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>