boards: nrf51dk/dongle: change SoC to nRF51822

The physical nRF51dk / nRF51dongle hardware contains a nRF51422 SoC.
In Zephyr, only the nRF51822 SoC, is implemented.

In Zephyr, the nRF51422 SoC is build as a nRF51822, therefore use the
nRF51822 SoC model for those kits instead on the non-existing nRF51422.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>