nrf51: show proper part number

Change-Id: I3e8169eb577888ee54541a1e864a8ad5a9a7cfd1
Signed-off-by: Jiří Pinkava <j-pi@seznam.cz>
Reviewed-on: http://openocd.zylin.com/3733


Tested-by: jenkins
Reviewed-by: Fredrik Hederstierna <fredrik@hederstierna.com>
Reviewed-by: Michael Dietz <mjdietzx@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>