tcl/board: Add board configuration for nRF51-DK, and nRF52-DK.

tcl/board: Add board configuration for nRF51-DK, and nRF52-DK.

Both boards use SEGGER interface.

nRF52 flash driver is in change 3511.

The board files were needed for transport select to SWD. The default
jtag transport didn't failover to SWD.

Change-Id: Id9c72aa88eeeb6a4e1999ee182f5284dbe535c05
Signed-off-by: Rick Foos <rfoos@solengtech.com>
Reviewed-on: http://openocd.zylin.com/3600
Tested-by: jenkins
Reviewed-by: Fredrik Hederstierna <fredrik.hederstierna@gmail.com>
Reviewed-by: Michael Dietz <mjdietzx@gmail.com>
Reviewed-by: Jiří Pinkava <j-pi@seznam.cz>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>