dts: common: nordic: nrf54l: Add clocks to cpu

Add clocks property to CPUs. nRF54Lx series is using hfpll as clock
source for CPU (and fast peripherals). CPU clock frequency can be
derived from frequency of the source clock so clock-frequency property
is removed from cpu as it is redundant.

nrfx/MDK expects that NRF_CONFIG_CPU_FREQ_MHZ define is set to correct
CPU frequency. Modified nrfx CMakeLists.txt to use clock frequency of
hfpll instead of CPU clock-frequency property.

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