PM / OPP: reuse of_parse_phandle()

We already have a better API to get the opp descriptor block's node from
cpu-node. Lets reuse that instead of creating our own routines for the
same stuff. That cleans the code a lot.

This also kills a check we had earlier (as we are using the generic API
now). Earlier we used to check if the operating-points-v2 property
contained multiple phandles instead of a single phandle.

Killing this check isn't an issue because, we only parse the first entry
with of_parse_phandle(). So, if a user passes multiple phandles, its
really his problem :)

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>