soc: sunxi: Move power-domain driver to the genpd dir

To simplify with maintenance let's move the sunxi 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: Chen-Yu Tsai <wens@csie.org>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: Samuel Holland <samuel@sholland.org>
Cc: <linux-sunxi@lists.linux.dev>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>