nrf52: Use nrf.h instead of *nrf*.h

Include nrf.h instead of directly using the specific header
required. This will be useful in the future when additional
Nordic ICs are supported.

JIRA: ZEP-377
Change-Id: I7a7257b0aaa5fa0a0d202322c366efbdd1d84458
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>