irqchip: Build IMX_MU_MSI only on ARM

Compile-testing IMX_MU_MSI on x86 without PCI_MSI support results in a
build failure:

drivers/gpio/gpio-sprd.c:8:
include/linux/gpio/driver.h:41:33: error: field 'msiinfo' has incomplete type
drivers/iommu/iommufd/viommu.c:4:
include/linux/msi.h:528:33: error: field 'alloc_info' has incomplete type

Tighten the dependency further to only allow compile testing on Arm.
This could be refined further to allow certain x86 configs.

This was submitted before to address a different build failure, which was
fixed differently, but the problem has now returned in a different form.

Fixes: 70afdab9 ("irqchip: Add IMX MU MSI controller driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250805160952.4006075-1-arnd@kernel.org
Link: https://lore.kernel.org/all/20221215164109.761427-1-arnd@kernel.org/