PM / OPP: Rename opp init/free table routines

free-table routines are opposite of init-table ones, and must be named
to make that clear. Opposite of 'init' is 'exit', but those doesn't suit
really well.

Replace 'init' with 'add' and 'free' with 'remove'.

Reported-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>