soc: mediatek: Move power-domain drivers to the genpd dir

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

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: <linux-mediatek@lists.infradead.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>