ext: Update and extend Nordic MDK header files

Update to version 8.6.0 and include nrf.h and the additional
files it requires for nRF52. This is so drivers targetting the
nRF Family can include nrf.h directly instead of *nrf5*.h

Origin: Nordic MDK 8.6.0
URL: http://www.nordicsemi.com/eng/Products/Bluetooth-Smart-Bluetooth-low-energy/nRF52832


Maintained-by: External

JIRA: ZEP-377
Change-Id: I71ad9828e1cf95e4d6ef09182a24f00e7d9f01ad
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>