arch: arm: nrf: Rename nrf5 SoC Family to nrf

Upcoming Nordic ICs that share many of the peripherals and architecture
with the currently supported nRF5x ones are no longer part of the nRF5
family. In order to accomodate that, rename the SoC family from nrf5 to
nrf, so that it can contain all of the members of the wider Nordic
family.

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