clock_control: nrf5_power: Use CMSIS NVIC APIs directly

Convert driver to use the CMSIS NVIC APIs rather than the internal ones
so we can remove them in the future.

Change-Id: I7bf3024ea4baf61f1e42bbbd5e4b0abfc6c04e6b
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>