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

To simplify with maintenance let's move the imx 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: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: <kernel@pengutronix.de>
Cc: <linux-imx@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>