dt-bindings: fsl: scu: add fallback compatible string for power domain

SCU power domain can be used in the same way by IMX8QXP and IMX8QM SoCs.
Let's add a "fsl,scu-pd" fallback compatible string to allow other SoCs
to reuse the common part.

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: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>