boards: gardena: Add smart garden radio module

This adds support for the SiM3U/Si4467 based SoM used on the GARDENA
smart Gateway [1] (art. no 19000 and 19005).

[1] https://github.com/husqvarnagroup/smart-garden-gateway-public



Signed-off-by: Michael Zimmermann <michael.zimmermann@grandcentrix.net>