irqchip: Add IMX MU MSI controller driver

The MU block found in a number of Freescale/NXP SoCs supports generating
IRQs by writing data to a register.

This enables the MU block to be used as a MSI controller, by leveraging
the platform-MSI API.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[maz: dropped pointless dma-iommu.h and of_pci.h includes]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220922161246.20586-4-Frank.Li@nxp.com