samples: boards: Merge nrf91/ with nrf52/ and rename it to nrf/

The following samples:
- boards/nrf91/nrfx
- boards/nrf52/power_mgr
are actually not specific to nRF91 and nRF52 Series SoCs, respectively,
hence the current naming of the above board/ subdirectories may be
misleading. Thus, use nrf/ directory for holding all nRF board specific
samples that apply to several boards.

Paths to the moved samples that are mentioned in their documentation
are also updated accordingly.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>