ext: Integrate Nordic SDK HAL files into Zephyr

Nordic Semiconductor's Software Development Kit's HAL layer
provides a set of low-level header and sourcefiles that give access
to the different hardware peripherals of Nordic ICs.

This patch includes the new files in the build and refactors the Kconfig
and Kbuild files in ext/hal/nordic to acommodate for the presence of the
new HAL layer.

Change-Id: Ie8e1a4c9fcc7e9058a9d16a2692ef1789603aa53
Signed-off-by: Wojciech Bober <wojciech.bober@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>