firmware: imx: add SCU power domain driver

Some i.MX SoCs contain a system controller that is responsible for
controlling the state of the IPs that are present. Communication
between the host processor running an OS and the system controller
happens through a SCU protocol. This patch adds SCU protocol based
power domains drivers.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Kevin Hilman <khilman@kernel.org>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>