ext: Update 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.

This new version adds support for the nRF52840 IC.

Origin: Nordic MDK 8.11.1
URL: http://www.nordicsemi.com/eng/Products/Bluetooth-low-energy


Maintained-by: External

JIRA: ZEP-1418

Change-Id: I76793f8f2bb1e588ed5244bd19a020bad021839b
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>