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.

Origin: Nordic MDK 8.13.0 (nRF5x MDK for Gcc with 3-clause BSD license)
URL: http://www.nordicsemi.com/eng/Products/Bluetooth-low-energy


Maintained-by: External

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>