drivers: watchdog: gecko: Add support for Gecko Series 2 SoC

The EFR32MG21 uses a different kind of clock for watchdogs.
It requires an additional peripheral-id parameter in the dts to get the
proper clock as it is based on the watchdog's ID i.e., cmuClock_WDOG#.

Signed-off-by: Steven Lemaire <steven.lemaire@zii.aero>