soc: nrf53: Add configuration options for HFXO/LFXO load capacitance

Add Kconfig options that allow configuration of optional internal
load capacitors for the high-frequency (HFXO) and low-frequency
(LFXO) crystal oscillators in nRF5340.
Default settings used for the new options are those that have been
in effect so far, i.e. external load capacitors for HFXO and 6 pF
internal capacitance for LFXO.

This commit also adds missing SOC_ENABLE_LFXO option dependency on
!TRUSTED_EXECUTION_NONSECURE.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>