drivers: clock_control: z_nrf_clock: add get_startup_time API

Implement vendor specific
z_nrf_clock_bt_ctlr_hf_get_startup_time_us() which gets the startup
time of the high frequency clock used for Bluetooth.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>