dt-bindings: irqchip: Describe the IMX MU block as a MSI controller

I.MX MU supports generating IRQs by writing to a register.

Describe its use as a MSI controller so that other blocks (such as a PCI EP)
can use it directly.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220922161246.20586-5-Frank.Li@nxp.com