boards: nordic: Set 64M CPU frequency for nrf54l20pdk and nrf54l09pdk

nrf54l20pdk and nrf54l09pdk must use 64 MHz for now. So far it was
done by using SOC_NRF54LX_SKIP_CLOCK_CONFIG Kconfig option which was
skipping oscillator configuration so that it was running the default
frequency (which is 64 MHz). This approach was a bit cryptic because
DT was indicating that CPU was running 128 MHz when actual frequency
was different (and it was relying on assumption that default frequency
is 64 MHz).

After adding hfpll as clock source for CPU Kconfig option can
be replaced with DT setting where actual frequency is correctly
indicated. Since hfpll is a clock source for fast peripherals (e.g.
TIMER00) it is possible to have single source of information
regarding frequency.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>