[nrf fromlist] boards: nordic: nrf54l20pdk: Add i2c to supported peripherals

Enable execution of i2c driver tests by adding i2c entry to
the list of supported peripherals.

Upstream PR #: 82965

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

boards/nordic/nrf54l20pdk/nrf54l20pdk_nrf54l20_cpuapp.yaml

@@ -15,3 +15,4 @@ flash: 449
 supported:
   - counter
   - gpio
+  - i2c