iommu/bcm2712: don't allow building as module

Since bcm2712-iommu{,-cache}.c doesn't have usual module descriptors
such as `MODULE_LICENSE`, configuring this as 'M' fails the build with
`ERROR: modpost: missing MODULE_LICENSE() in <...>/bcm2712-iommu.o`.
Since it seems like the code is not intended to be built as a module
anyway (it registers the driver with `builtin_platform_driver()`), don't
allow building this code as a module.

Signed-off-by: Ratchanan Srirattanamet <peathot@hotmail.com>