PM / Domain: Add struct device to genpd

The power-domain core would be using the OPP core going forward and the
OPP core has the basic requirement of a device structure for its working.

Add a struct device to the genpd structure. This doesn't register the
device with device core as the "dev" pointer is mostly used by the OPP
core as a cookie for now and registering the device is not mandatory.

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