boards: arm: Add support for the Nordic pca10056 PDK

With the appearance of the nRF52840 IC a new Preview Development Kit
(PDK) board has been introduced. This patch adds basic support for this
new board.

JIRA: ZEP-1418

Change-Id: If5845e75312ec756b968e595e5dc31c4c9624be2
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>