PM / Domains: Rename genpd virtual devices as virt_dev

There are several struct device instances that genpd core handles. The
most common one is the consumer device structure, which is named
(correctly) as "dev" within genpd core. The second one is the genpd's
device structure, referenced as genpd->dev. The third one is the virtual
device structures created by the genpd core to represent the consumer
device for multiple power domain case, currently named as genpd_dev. The
naming of these virtual devices isn't very clear or readable and it
looks more like the genpd->dev.

Rename the virtual device instances within the genpd core as "virt_dev".

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>