MAINTAINERS: Also assign siwx91x drivers to Silabs

Usually, drivers for siwx91x are named "silabs_siwx91x". However, there
are a few exceptions:
  - **/Kconfig.siwx91x
  - drivers/wifi/siwx91x

Let's ensure the reviewers for these files are properly assigned.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>