18756: drivers/usbdev_synopsys_dwc2: add EFM32 support r=chrysn a=gschorcht
### Contribution description
This PR provides the changes for the Synopsys USB OTG IP core DWC2 driver for EFM32 MCUs. It also provides the changes of the board definition for `stk3600` and `stk3700` for testing.
### Testing procedure
`tests/usbus_hid` should work on the EFM32 boards `stk3600` (EFM32LG family) and `stk3700` (EFM32GG family).
It is already tested for a `sltb009a` board (EFM32GG12 family).
### Issues/PRs references
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
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