dt-bindings: firmware: Add fsl,scu yaml file

In order to replace the fsl,scu txt file from bindings/arm/freescale,
we need to split it between the right subsystems. This patch adds the
fsl,scu.yaml in the firmware bindings folder. This one is only for
the main SCU node. The old txt file will be removed only after all
the child nodes have been properly switch to yaml.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>