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

To simplify with maintenance let's move the starfive 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: Walker Chen <walker.chen@starfivetech.com>
Cc: Conor Dooley <conor@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>