clock_control: nrf5: Add USBD power event support for USB enumeration

nRF52840 USBD requires power events to start USB enumeration.

Add APIs to enable/disable the power events, read the USB VBUS detect
and output power ready statuses which can then be used by the USB driver
in the enumeration sequence.

Signed-off-by: Sundar Subramaniyan <sundar.subramaniyan@gmail.com>