boards: nrf51: Suppress DTC warnings about duplicate unit-address

Prevent "unique_unit_address_if_enabled" warnings from being reported
for nRF51 Series SoCs, where certain nodes need to be enabled with
the same base addresses. These are:
- power@40000000 & clock@40000000 & mpu@40000000

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