opp: Not all power-domains are scalable

A device may have multiple power-domains and not all of them may be
scalable (i.e. support performance states). But
dev_pm_opp_attach_genpd() doesn't take that into account currently.

Fix that by not verifying the names argument with "power-domain-names"
DT property and finding the index into the required-opps array. The
names argument will anyway get verified later on when we call
dev_pm_domain_attach_by_name().

Fixes: 6319aee1

 ("opp: Attach genpds to devices from within OPP core")
Reported-by: Rajendra Nayak <rnayak@codeaurora.org>
Tested-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>