Silabs has released the new EFR32MG12 parts which are supported in Simplicity Studio V4 via the Software Update and new stack installation process, but if I want to compile or debug directly in IAR, how do I get the new parts?
In the Silabs Portal under Software Releases, find the version of IAR you are using, for example IAR Embedded Workbench for ARM, v7.80.2(sorting Release Name alphabetically under "I" helps).
In the Associated Content, find the IAR-EFR32xG12x-part-support-5.1.0and download the zipfile. Inside it, there is another zipfile that contains the sub-family you are looking for. In specific, most Mesh users will want the IAR-EFR32-Mighty-5.1.0.zip file. Copy this file into your IAR installation, in the top level of the EWARM version you are using. In this case, C:\Program Files (x86)\IAR Systems\Embedded Workbench 7.5\ -- it goes wherever the armdirectory exists.
Make sure the IAR application is closed. Unzip the file "here". This will place the contents into the correct subdirectories. When you restart, you will be able to specify the part used when you open the eww file by right-clicking on the project name, and picking Options... > General Options > Target > Device and pulling the menu down to SiliconLaboratories EFR32MG12[the rest of your part number here].