ext: Import Nordic MDK header files

Nordic Semiconductor's Microcontroller Development Kit
provides a set of low-level header files that describe
the different hardware registers and peripherals of
Nordic ICs.

Origin: Nordic MDK 8.5.0
URL: http://www.nordicsemi.com/eng/Products/Bluetooth-Smart-Bluetooth-low-energy/nRF52832


Maintained-by: External

JIRA: ZEP-377
Change-Id: Ia28fbaf7333a84ea7462fb7fd7a5d06661a6fce0
Signed-off-by: Glenn Ruben Bakke <glenn.ruben.bakke@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Håkon Alseth <haakon.alseth@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>