opp: Allocate genpd_virt_devs from dev_pm_opp_attach_genpd()

opp: Allocate genpd_virt_devs from dev_pm_opp_attach_genpd()

Currently the space for the array of virtual devices is allocated along
with the OPP table, but that isn't going to work well from now onwards.
For single power domain case, a driver can either use the original
device structure for setting the performance state (if genpd attached
with dev_pm_domain_attach()) or use the virtual device structure (if
genpd attached with dev_pm_domain_attach_by_name(), which returns the
virtual device) and so we can't know in advance if we are going to need
genpd_virt_devs array or not.

Lets delay the allocation a bit and do it along with
dev_pm_opp_attach_genpd() rather. The deallocation is done from
dev_pm_opp_detach_genpd().

Tested-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>