dt-bindings: fsl: scu: update power domain binding

Update the power domain binding with #power-domain-cells 1 format.
The first cell can be a global SCU power domain and the 2nd cell
the device ID. With this approach, we may remove all the sub power
domain nodes from device tree which can relief the device tree a lot.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>