drivers: ieee802154_nrf5: cabability IEE802154_OPENTHREAD_HW_CST

The new Kconfig option `IEEE802154_NRF5_CST_ENDPOINT` is added.
When it is enabled, then capability `IEE802154_OPENTHREAD_HW_CST`
is supported by the ieee802154_nrf5 driver and allows to set the
CST period and CST expected transmission time point.
This feature is an OpenThread-specific extention to the
ieee802154_nrf5 driver.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>
Co-authored-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>