samples: drivers: watchdog: Add overlay files for nrf54l15/cpuflpr

Both nrf54l15pdk/cpuflpr and nrf54l15pdk/cpuflpr_xip have
watchdog on the list of supported periperals.
As a result, Twister tries to build samples/drivers/watchdog
for these two targets.
Since overlays are missing, build is failing.

Add missing overlay files.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>