soc: rockchip: Mover power-domain driver to the genpd dir

To simplify with maintenance let's move the rockchip power-domain driver to
the new genpd directory. Going forward, patches are intended to be managed
through a separate git tree, according to MAINTAINERS.

Cc: Heiko Stuebner <heiko@sntech.de>
Cc: <linux-rockchip@lists.infradead.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>