PM / Domains: Add generic data pointer to genpd data struct

Add a void *data pointer to struct generic_pm_domain_data. Because this
exists for each device associated with a genpd it will allow us to
assign per-device data if needed on a platform for control of that
specific device.

Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>