cpu/samd21: default 1kHz clock to same source as 32kHz clock
cpu/samd21: default 1kHz clock to same source as 32kHz clock

The split between GEN2_ULP32K and GEN3_ULP32K was introduced to fix
a failure in tests/periph_wdt when the external oscillator was used.

By not running the external oscillator on demand, the failure can no
longer be observed, so default GEN3_ULP32K to GEN2_ULP32K.

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