dt/bindings: Add bindings for Layerscape SCFG MSI

Some Layerscape SoCs use a simple MSI controller implementation.
It contains only two SCFG register to trigger and describe a
group 32 MSI interrupts. The patch adds bindings to describe
the controller.

Signed-off-by: Minghuan Lian <Minghuan.Lian@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>