tests: drivers: watchdog: exclude platforms with SAM WDT from testing

On watchdog-triggered reboot, the SAM platforms
reset RAM, so the wdt_basic_api cannot be completed
successfully, as it relies on RAM retention (relies
on variables stored in RAM retaining their values
across different boot cycles). Exclude the platforms
with the SAM Watchdog from this test.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>