boards: nrf52: Suppress DTC warnings about duplicate unit-address

Prevent "unique_unit_address_if_enabled" warnings from being reported
for nRF52 Series SoCs, where certain nodes need to be enabled with
the same base addresses. These can be (depending on a given SoC):
- power@40000000 & clock@40000000
- power@40000000 & clock@40000000 & bprot@40000000
- acl@4001e000 & flash-controller@4001e000

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