arch: arm: Refactor and cleanup nRF52 series arch code

In preparation for the upcoming nRF52840 support, this patch refactors
the nRF52 series support code to allow for future members of the IC
family to be added, while keeping everything that is common together.

JIRA: ZEP-1418

Change-Id: I4200064ca888d72ba0b8629ce94e4ea6384099ad
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>